Fix user authentication token not being sent to the server (#238)

## What is the goal of this PR?

We've fixed two bugs regarding the user authentication token.

## What are the changes implemented in this PR?

1. Fix the bug where the token isn't unpacked properly when received
2. Fix the bug where username isn't sent along with the token

Author: Ganeshwara Hananda

typedb/connection/cluster/server_client.py

@@ -76,4 +76,4 @@ def __call__(self, context, callback):
         if token is None:
             callback((('username', self._credential.username()), ('password', self._credential.password())), None)
         else:
-            callback((('token', token)), None)
+            callback((('username', self._credential.username()), ('token', token)), None)

typedb/connection/cluster/stub.py

@@ -49,7 +49,8 @@ def __init__(self, channel: Channel, credential: TypeDBCredential):
         self._cluster_stub = cluster_service_proto.TypeDBClusterStub(channel)
         self._token = None
         try:
-            self._token = self._cluster_stub.user_token(cluster_user_token_req(self._credential.username()))
+            res = self._cluster_stub.user_token(cluster_user_token_req(self._credential.username()))
+            self._token = res.token
         except RpcError as e:
             e2 = TypeDBClientException.of_rpc(e)
             if e2.error_message is not None and e2.error_message is not UNABLE_TO_CONNECT: