add register_namespace property

[TomW1605]

this is to allow the registration of class based namespaces as described in the docs: python-socketio: Class-Based Namespaces

simply add this to the end of socket_manager.py

@property
def register_namespace(self):
    return self._sio.register_namespace

[TomW1605]

any chance this could be added? happy to do a PR with it