docs: add instructions for using workload identity federation (#564)

* docs: add instructions for using workload identity federation

* docs: address review comments

README.md

@@ -106,6 +106,12 @@ access tokens
 - `ServiceAccountJwtAccessCredentials`: credentials for a Service Account - use JSON Web Token (JWT)
 directly in the request metadata to provide authorization
 - `UserCredentials`: credentials for a user identity and consent
+- `ExternalAccountCredentials`: base class for credentials using workload identity federation to 
+access Google Cloud resources from non-Google Cloud platforms
+- `IdentityPoolCredentials`: credentials using workload identity federation to access Google Cloud 
+resources from Microsoft Azure or any identity provider that supports OpenID Connect (OIDC)
+- `AwsCredentials`: credentials using workload identity federation to access Google Cloud resources 
+from Amazon Web Services (AWS)
 
 To get Application Default Credentials use `GoogleCredentials.getApplicationDefault()` or
 `GoogleCredentials.getApplicationDefault(HttpTransportFactory)`. These methods return the
@@ -159,6 +165,219 @@ for (Bucket b : storage_service.list().iterateAll())
     System.out.println(b); 
 ```
 
+### Workload Identity Federation
+
+Using workload identity federation, your application can access Google Cloud resources from
+Amazon Web Services (AWS), Microsoft Azure, or any identity provider that supports OpenID Connect
+(OIDC).
+
+Traditionally, applications running outside Google Cloud have used service account keys to access
+Google Cloud resources. Using identity federation, your workload can impersonate a service account.
+This lets the external workload access Google Cloud resources directly, eliminating the maintenance
+and security burden associated with service account keys.
+
+#### Accessing resources from AWS
+
+In order to access Google Cloud resources from Amazon Web Services (AWS), the following requirements
+are needed:
+- A workload identity pool needs to be created.
+- AWS needs to be added as an identity provider in the workload identity pool (the Google [organization policy](https://cloud.google.com/iam/docs/manage-workload-identity-pools-providers#restrict) needs to allow federation from AWS).
+- Permission to impersonate a service account needs to be granted to the external identity.
+
+Follow the detailed [instructions](https://cloud.google.com/iam/docs/access-resources-aws) on how to
+configure workload identity federation from AWS.
+
+After configuring the AWS provider to impersonate a service account, a credential configuration file
+needs to be generated. Unlike service account credential files, the generated credential
+configuration file contains non-sensitive metadata to instruct the library on how to
+retrieve external subject tokens and exchange them for service account access tokens.
+The configuration file can be generated by using the [gcloud CLI](https://cloud.google.com/sdk/).
+
+To generate the AWS workload identity configuration, run the following command:
+
+```bash
+# Generate an AWS configuration file.
+gcloud iam workload-identity-pools create-cred-config \
+    projects/$PROJECT_NUMBER/locations/global/workloadIdentityPools/$POOL_ID/providers/$AWS_PROVIDER_ID \
+    --service-account $SERVICE_ACCOUNT_EMAIL \
+    --aws \
+    --output-file /path/to/generated/config.json
+```
+
+Where the following variables need to be substituted:
+- `$PROJECT_NUMBER`: The Google Cloud project number.
+- `$POOL_ID`: The workload identity pool ID.
+- `$AWS_PROVIDER_ID`: The AWS provider ID.
+- `$SERVICE_ACCOUNT_EMAIL`: The email of the service account to impersonate.
+
+This generates the configuration file in the specified output file.
+
+You can now [use the Auth library](#using-external-identities) to call Google Cloud
+resources from AWS.
+
+#### Access resources from Microsoft Azure
+
+In order to access Google Cloud resources from Microsoft Azure, the following requirements are
+needed:
+- A workload identity pool needs to be created.
+- Azure needs to be added as an identity provider in the workload identity pool (the Google [organization policy](https://cloud.google.com/iam/docs/manage-workload-identity-pools-providers#restrict) needs to allow federation from Azure).
+- The Azure tenant needs to be configured for identity federation.
+- Permission to impersonate a service account needs to be granted to the external identity.
+
+Follow the detailed [instructions](https://cloud.google.com/iam/docs/access-resources-azure) on how
+to configure workload identity federation from Microsoft Azure.
+
+After configuring the Azure provider to impersonate a service account, a credential configuration
+file needs to be generated. Unlike service account credential files, the generated credential
+configuration file contains non-sensitive metadata to instruct the library on how to
+retrieve external subject tokens and exchange them for service account access tokens.
+The configuration file can be generated by using the [gcloud CLI](https://cloud.google.com/sdk/).
+
+To generate the Azure workload identity configuration, run the following command:
+
+```bash
+# Generate an Azure configuration file.
+gcloud iam workload-identity-pools create-cred-config \
+    projects/$PROJECT_NUMBER/locations/global/workloadIdentityPools/$POOL_ID/providers/$AZURE_PROVIDER_ID \
+    --service-account $SERVICE_ACCOUNT_EMAIL \
+    --azure \
+    --output-file /path/to/generated/config.json
+```
+
+Where the following variables need to be substituted:
+- `$PROJECT_NUMBER`: The Google Cloud project number.
+- `$POOL_ID`: The workload identity pool ID.
+- `$AZURE_PROVIDER_ID`: The Azure provider ID.
+- `$SERVICE_ACCOUNT_EMAIL`: The email of the service account to impersonate.
+
+This generates the configuration file in the specified output file.
+
+You can now [use the Auth library](#using-external-identities) to call Google Cloud
+resources from Azure.
+
+#### Accessing resources from an OIDC identity provider
+
+In order to access Google Cloud resources from an identity provider that supports [OpenID Connect (OIDC)](https://openid.net/connect/), the following requirements are needed:
+- A workload identity pool needs to be created.
+- An OIDC identity provider needs to be added in the workload identity pool (the Google [organization policy](https://cloud.google.com/iam/docs/manage-workload-identity-pools-providers#restrict) needs to allow federation from the identity provider).
+- Permission to impersonate a service account needs to be granted to the external identity.
+
+Follow the detailed [instructions](https://cloud.google.com/iam/docs/access-resources-oidc) on how
+to configure workload identity federation from an OIDC identity provider.
+
+After configuring the OIDC provider to impersonate a service account, a credential configuration
+file needs to be generated. Unlike service account credential files, the generated credential
+configuration file contains non-sensitive metadata to instruct the library on how to
+retrieve external subject tokens and exchange them for service account access tokens.
+The configuration file can be generated by using the [gcloud CLI](https://cloud.google.com/sdk/).
+
+For OIDC providers, the Auth library can retrieve OIDC tokens either from a local file location
+(file-sourced credentials) or from a local server (URL-sourced credentials).
+
+**File-sourced credentials**
+For file-sourced credentials, a background process needs to be continuously refreshing the file
+location with a new OIDC token prior to expiration. For tokens with one hour lifetimes, the token
+needs to be updated in the file every hour. The token can be stored directly as plain text or in
+JSON format.
+
+To generate a file-sourced OIDC configuration, run the following command:
+
+```bash
+# Generate an OIDC configuration file for file-sourced credentials.
+gcloud iam workload-identity-pools create-cred-config \
+    projects/$PROJECT_NUMBER/locations/global/workloadIdentityPools/$POOL_ID/providers/$OIDC_PROVIDER_ID \
+    --service-account $SERVICE_ACCOUNT_EMAIL \
+    --credential-source-file $PATH_TO_OIDC_ID_TOKEN \
+    # Optional arguments for file types. Default is "text":
+    # --credential-source-type "json" \
+    # Optional argument for the field that contains the OIDC credential.
+    # This is required for json.
+    # --credential-source-field-name "id_token" \
+    --output-file /path/to/generated/config.json
+```
+
+Where the following variables need to be substituted:
+- `$PROJECT_NUMBER`: The Google Cloud project number.
+- `$POOL_ID`: The workload identity pool ID.
+- `$OIDC_PROVIDER_ID`: The OIDC provider ID.
+- `$SERVICE_ACCOUNT_EMAIL`: The email of the service account to impersonate.
+- `$PATH_TO_OIDC_ID_TOKEN`: The file path used to retrieve the OIDC token.
+
+This generates the configuration file in the specified output file.
+
+**URL-sourced credentials**
+For URL-sourced credentials, a local server needs to host a GET endpoint to return the OIDC token.
+The response can be in plain text or JSON. Additional required request headers can also be
+specified.
+
+To generate a URL-sourced OIDC workload identity configuration, run the following command:
+
+```bash
+# Generate an OIDC configuration file for URL-sourced credentials.
+gcloud iam workload-identity-pools create-cred-config \
+    projects/$PROJECT_NUMBER/locations/global/workloadIdentityPools/$POOL_ID/providers/$OIDC_PROVIDER_ID \
+    --service-account $SERVICE_ACCOUNT_EMAIL \
+    --credential-source-url $URL_TO_GET_OIDC_TOKEN \
+    --credential-source-headers $HEADER_KEY=$HEADER_VALUE \
+    # Optional arguments for file types. Default is "text":
+    # --credential-source-type "json" \
+    # Optional argument for the field that contains the OIDC credential.
+    # This is required for json.
+    # --credential-source-field-name "id_token" \
+    --output-file /path/to/generated/config.json
+```
+
+Where the following variables need to be substituted:
+- `$PROJECT_NUMBER`: The Google Cloud project number.
+- `$POOL_ID`: The workload identity pool ID.
+- `$OIDC_PROVIDER_ID`: The OIDC provider ID.
+- `$SERVICE_ACCOUNT_EMAIL`: The email of the service account to impersonate.
+- `$URL_TO_GET_OIDC_TOKEN`: The URL of the local server endpoint to call to retrieve the OIDC token.
+- `$HEADER_KEY` and `$HEADER_VALUE`: The additional header key/value pairs to pass along the GET
+request to `$URL_TO_GET_OIDC_TOKEN`, e.g. `Metadata-Flavor=Google`.
+
+You can now [use the Auth library](#using-external-identities) to call Google Cloud
+resources from an OIDC provider.
+
+#### Using External Identities
+
+External identities (AWS, Azure, and OIDC-based providers) can be used with
+`Application Default Credentials`. In order to use external identities with Application Default
+Credentials, you need to generate the JSON credentials configuration file for your external identity
+as described above. Once generated, store the path to this file in the
+`GOOGLE_APPLICATION_CREDENTIALS` environment variable.
+
+```bash
+export GOOGLE_APPLICATION_CREDENTIALS=/path/to/config.json
+```
+
+The library can now choose the right type of client and initialize credentials from the context
+provided in the configuration file.
+
+```java
+GoogleCredentials googleCredentials = GoogleCredentials.getApplicationDefault();
+
+String projectId = "your-project-id";
+String url = "https://storage.googleapis.com/storage/v1/b?project=" + projectId;
+
+HttpCredentialsAdapter credentialsAdapter = new HttpCredentialsAdapter(googleCredentials);
+HttpRequestFactory requestFactory = new NetHttpTransport().createRequestFactory(credentialsAdapter);
+HttpRequest request = requestFactory.buildGetRequest(new GenericUrl(url));
+
+JsonObjectParser parser = new JsonObjectParser(GsonFactory.getDefaultInstance());
+request.setParser(parser);
+
+HttpResponse response = request.execute();
+System.out.println(response.parseAsString());
+```
+
+You can also explicitly initialize external account clients using the generated configuration file.
+
+```java
+ExternalAccountCredentials credentials = 
+    ExternalAccountCredentials.fromStream(new FileInputStream("/path/to/credentials.json"));
+```
+
 ## Configuring a Proxy
 
 For HTTP clients, a basic proxy can be configured by using `http.proxyHost` and related system properties as documented