feat(experimentalIdentityAndAuth): Add generic @aws.auth#sigv4 support

[syall]

Issue #, if available:

N/A.

Description of changes:

NOTE: All of these are under the experimentalIdentityAndAuth feature flag

NOTE: Dependent on #881

• Adding config properties region and credentials:

  /**
   * The AWS region to which this client will send requests.
   */
  region?: string | __Provider<string>;
  
  /**
   * The credentials used to sign requests.
   */
  credentials?: AwsCredentialIdentity | AwsCredentialIdentityProvider;

• @smithy/experimental-identity-and-auth package: add the temporary SigV4Signer implementation (still need to resolve numerous signingProperties for backward compatibility)
• Add back operations that support @aws.auth#sigv4 in the generic client test

Testing:

• @aws.auth#sigv4 client codegen diff given the following model changes: https://gist.github.com/syall/a595c7738c380176a528ff29016f1c2c#file-awsauthsigv4-trait-client-diff

   //   auth scheme support is implemented
   // @httpApiKeyAuth(name: "X-Api-Key", in: "header")
   // @httpBearerAuth
  -// @sigv4(name: "weather")
  +@sigv4(name: "weather")
   // @auth([sigv4])
   @paginated(inputToken: "nextToken", outputToken: "nextToken", pageSize: "pageSize")
   service Weather {
  @@ -28,10 +28,12 @@ service Weather {
           // experimentalIdentityAndAuth
           // OnlyHttpApiKeyAuth
           // OnlyHttpBearerAuth
  +        OnlySigv4Auth
           // OnlyHttpApiKeyAndBearerAuth
           // OnlyHttpApiKeyAndBearerAuthReversed
           // OnlyHttpApiKeyAuthOptional
  -        // SameAsService
  +        OnlySigv4AuthOptional
  +        SameAsService
       ]
   }

• No codegen diff for aws-sdk-js-v3

If one or more of the packages in the /packages directory has been modified, be sure yarn changeset add has been run and its output has
been committed and included in this pull request. See CONTRIBUTING.md.

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.