feat(firebase-x): add getId (#3388)

* feat(firebase-x): add getId

add getId to get app instance id. fixes #3378

* Update index.ts

src/@ionic-native/plugins/firebase-x/index.ts

@@ -119,6 +119,15 @@ export class FirebaseX extends IonicNativePlugin {
     return;
   }
 
+  /**
+   * Get the app instance ID (an constant ID which persists as long as the app is not uninstalled/reinstalled)
+   * @return {Promise<null | string>} Note that ID will be null if it has not been established yet
+   */
+  @Cordova()
+  getId(): Promise<null | string> {
+    return;
+  }
+
   /**
    * Get notified when a token is refreshed.
    * @return {Observable<any>}