feat userver: expose topic client in testsuite

Tests: затронут только тестсьют
commit_hash:1d064aacda9fd95ee9f162123f8869eb4121f6e8

Author: leonidlazarev

testsuite/pytest_plugins/pytest_userver/plugins/ydb/client.py

@@ -12,6 +12,10 @@ def __init__(self, endpoint, database):
     def execute(self, query):
         return self._session.transaction().execute(query, commit_tx=True)
 
+    @property
+    def topic_client(self):
+        return self._driver.topic_client
+
     @property
     def session(self):
         return self._session