feat(llc, persistence): add support for open and close persistence via client.

Signed-off-by: xsahil03x <[email protected]>

packages/stream_chat/test/src/db/chat_persistence_client_test.dart

@@ -15,6 +15,9 @@ class TestPersistenceClient extends ChatPersistenceClient {
   @override
   bool get isConnected => throw UnimplementedError();
 
+  @override
+  String? get userId => throw UnimplementedError();
+
   @override
   Future<void> connect(String userId) => throw UnimplementedError();