[App Config] updateSyncToken method (Azure#14507)

### TODOs
- [x] Add the new `updateSyncToken` method to `AppConfigurationClient` 
  (Picked up pieces from @richardpark-msft's work here Azure#14342.)
- [x] Add a unit test
- [x] ~~Hoping to add a live test by leveraging the eventgrid SDK~~
- [x] Changelog
- [x] Hold off merging - wait for 1.1.1 to be released
- [x] What's the new version number for preview? - `1.2.0-beta.1` fine?
  
Reference: PR from .NET Azure/azure-sdk-for-net#18487

sdk/appconfiguration/app-configuration/CHANGELOG.md

@@ -1,5 +1,10 @@
 # Release History
 
+## 1.2.0-beta.1 (Unreleased)
+
+- Added `updateSyncToken` method to `AppConfigurationClient` to be able to provide external synchronization tokens.
+  [PR #14507](https://github.com/Azure/azure-sdk-for-js/pull/14507)
+
 ## 1.1.1 (2021-03-25)
 
 - Fix issues with `select`ing fields to be returned from `listConfigurationSettings`, `listConfigurationRevisions`
@@ -13,7 +18,7 @@
 ## 1.0.1 (2020-02-19)
 
 - The underlying filter behavior has changed for `listConfigurationSettings` and `listRevisions`.
-  Inline documentation has been revised to accomodate it.
+  Inline documentation has been revised to accommodate it.
 
 ## 1.0.0 (2020-01-06)
 

sdk/appconfiguration/app-configuration/review/app-configuration.api.md

@@ -33,7 +33,8 @@ export class AppConfigurationClient {
     listRevisions(options?: ListRevisionsOptions): PagedAsyncIterableIterator<ConfigurationSetting, ListRevisionsPage>;
     setConfigurationSetting(configurationSetting: SetConfigurationSettingParam, options?: SetConfigurationSettingOptions): Promise<SetConfigurationSettingResponse>;
     setReadOnly(id: ConfigurationSettingId, readOnly: boolean, options?: SetReadOnlyOptions): Promise<SetReadOnlyResponse>;
-    }
+    updateSyncToken(syncToken: string): void;
+}
 
 // @public
 export interface AppConfigurationClientOptions {

sdk/appconfiguration/app-configuration/src/appConfigurationClient.ts

@@ -115,6 +115,7 @@ export interface InternalAppConfigurationClientOptions extends AppConfigurationC
  */
 export class AppConfigurationClient {
   private client: AppConfiguration;
+  private _syncTokens: SyncTokens;
   // (for tests)
   private _trace = traceFromTracingHelpers;
 
@@ -164,13 +165,13 @@ export class AppConfigurationClient {
       }
     }
 
-    const syncTokens = appConfigOptions.syncTokens || new SyncTokens();
+    this._syncTokens = appConfigOptions.syncTokens || new SyncTokens();
 
     this.client = new AppConfiguration(
       appConfigCredential,
       appConfigEndpoint,
       apiVersion,
-      getGeneratedClientOptions(appConfigEndpoint, syncTokens, appConfigOptions)
+      getGeneratedClientOptions(appConfigEndpoint, this._syncTokens, appConfigOptions)
     );
   }
 
@@ -496,8 +497,16 @@ export class AppConfigurationClient {
       }
     });
   }
-}
 
+  /**
+   * Adds an external synchronization token to ensure service requests receive up-to-date values.
+   *
+   * @param syncToken The synchronization token value.
+   */
+  updateSyncToken(syncToken: string): void {
+    this._syncTokens.addSyncTokenFromHeaderValue(syncToken);
+  }
+}
 /**
  * Gets the options for the generated AppConfigurationClient
  * @internal

sdk/appconfiguration/app-configuration/test/internal/http.spec.ts

@@ -292,6 +292,38 @@ describe("http request related tests", function() {
       assert.equal(syncTokens.getSyncTokenHeaderValue(), "clearReadOnly=value");
     });
   });
+
+  describe("syncToken", async () => {
+    it("update sync token", async () => {
+      const syncTokens = new SyncTokens();
+      syncTokens.addSyncTokenFromHeaderValue("a=value;sn=0");
+      const client = new AppConfigurationClient(
+        "Endpoint=https://endpoint.azconfig.io;Id=abc;Secret=123",
+        { syncTokens } as InternalAppConfigurationClientOptions
+      );
+      assert.equal(
+        syncTokens["_currentSyncTokens"].size,
+        1,
+        "Unexpected number of syncTokens before the `update` call"
+      );
+      client.updateSyncToken("b=value;sn=3");
+      assert.equal(
+        syncTokens["_currentSyncTokens"].size,
+        2,
+        "Unexpected number of syncTokens after the `update` call"
+      );
+      assert.deepEqual(
+        syncTokens["_currentSyncTokens"].get("a"),
+        { id: "a", value: "value", sequenceNumber: 0 },
+        "Unexpected object present for key `a`"
+      );
+      assert.deepEqual(
+        syncTokens["_currentSyncTokens"].get("b"),
+        { id: "b", value: "value", sequenceNumber: 3 },
+        "Unexpected object present for key `b`"
+      );
+    });
+  });
 });
 
 function splitAndSort(syncTokens: string | undefined): string {