Create X protocol messages with an API that current protobuf still has.

Upstream pins protobuf==5.29.4. MessageFactory.GetPrototype() no longer exists
in the protobuf that Gentoo ships (checked with 7.35.1), so without the
optional _mysqlxpb C extension creating any message fails with AttributeError.
GetMessageClass() is its documented replacement. Fedora avoids this path by
building the C extension.

--- a/mysqlx-connector-python/lib/mysqlx/protobuf/__init__.py
+++ b/mysqlx-connector-python/lib/mysqlx/protobuf/__init__.py
@@ -285,7 +285,7 @@
             Returns:
                 object: Protobuf message.
             """
-            cls = _mysqlxpb_pure.factory.GetPrototype(
+            cls = message_factory.GetMessageClass(
                 _DESCRIPTOR_POOL.FindMessageTypeByName(name)
             )
             return cls()