Wildcard / catch-all namespace for event handlers

[mooomooo]

Is your feature request related to a problem? Please describe.
It's possible to accept connections on any namespace by specifying namespaces='*' in either Server init, but you can't actually handle arbitrary namespaces: unless a namespace is explicitly defined by an event handler, any client packets using it will get silently dropped.

Describe the solution you'd like
Allow specifying an event handler with namespace='*' to be a catch-all handler in the absence of an explicit namespace match. In that case, the handler should also be given an explicit namespace parameter with the namespace from the client packet.

Additional context
Such a thing is already implemented for catch-all event handlers in the server's _trigger_event (e.g.

python-socketio/src/socketio/server.py

Lines 610 to 611 in 6967eca

	elif event not in self.reserved_events and \
	'*' in self.handlers[namespace]:

), so similar logic could probably be used at the namespace level as well.

This is could perhaps allow a high level approach to #1166 with less library overhead since there'd be no regex comparison (though it could nonetheless be done by a user if so desired).

[miguelgrinberg]

Yes, in principle I like this idea as it is consistent with previous work for event handlers. The event handlers attached to the catch all namespace will need to receive the namespace as an additional argument, so that they can implement logic that depends on which namespace the client used.

Would you like to give this idea a try and send a PR?