Support an external WebSocket object

[davidbrochart]

Is your feature request related to a problem? Please describe.
Document providers like y-webrtc use lib0/websocket for connecting e.g. to a signaling server, which doesn't use any authentication system. But in practice, connections to the signaling server should be protected.

Describe the solution you'd like
Would it be possible to pass an external WebSocket object, that would be pre-configured to make an authenticated request?

Describe alternatives you've considered
This seems to be the most general/simply solution, I'm not thinking of alternatives.

Additional context
I have forked y-webrtc in jupyter-webrtc-provider. Maybe I could remove the dependency on lib0 for the WebSocket. Do you think that would be a better solution?

[dmonad]

Options that I would be fine with:

• adding a custom WebSocket instance
• adding a "protocol message" parameter, that sends the auth token via protocol message (recommended, if you can't use cookies)
• adding an option to configure url parameters
• forking does make sense - you would just lose some composability.

The code already exists in y-websocket and could be moved to lib0.