Signaling through XMPP (or other protocols?)

[zpostfacto]

Some sort of signaling service is needed for P2P connections. (See README_P2P.md). Steamworks provides this services for games on Steam, but we need a solution for the opensource version. We should support other open-source standards. XMPP in particular seems appropriate, but perhaps there are others? (Maybe Tailscale DERP ?)

This is probably a great task for somebody motivated. It would not require modifying any of existing files, the code is already organized for this to be customizable, and there is an example that demonstrates what needs to be done.

We would welcome your additions to this repository, or if you want it to be your own project, we would be happy to link to it, assuming it meets some basic requirements.