Fleshed out session management in registration v2.

libsignal-service/src/main/java/org/whispersystems/signalservice/api/registration/RegistrationApi.kt

@@ -30,6 +30,15 @@ class RegistrationApi(
     }
   }
 
+  /**
+   * Retrieve current status of a registration session.
+   */
+  fun getRegistrationSessionStatus(sessionId: String): NetworkResult<RegistrationSessionMetadataResponse> {
+    return NetworkResult.fromFetch {
+      pushServiceSocket.getSessionStatus(sessionId)
+    }
+  }
+
   /**
    * Submit an FCM token to the service as proof that this is an honest user attempting to register.
    */