Handle abrupt player disconnection

[pierd]

Explicit disconnects are handled properly but if a connection times out or ambient is simply killed then the player is still seen as connected.

Add an explicit player disconnected callback
OR
Add handling inside proxied connection and report an error there to let Ambient handle disconnection

[ten3roberts]

You could have the proxy send a Disconnect frame on behalf of the timed out user

See: https://github.com/AmbientRun/Ambient/blob/f4e68976e77dac06268883acad0bcd82aaf7f493/crates/network/src/proto/mod.rs#L10

[ten3roberts]

Feel free to add more variants to that enum if you need, it will be backwards compatible anyway as long as they are added after the existing variants

[pierd]

I'd prefer proxy to be more transparent and not to rely too much on Ambient protocol.