Steam Peer to Peer connection/disconnection events

[SueHeir]

Hello again, I noticed steam peer to peer connections will allow joining a server not started and doesn't disconnect disconnected clients when the server is stopped.

Might be related to https://github.com/cBournhonesque/lightyear/issues/243

I wanted to brainstorm some ideas on temp fixes. If steam's "shutdown" api pretty much does nothing. Maybe we could pass whether the server is running into ConnectionRequestHandler (not sure if that is an option) to deny the request. Perhaps we can use lobby IDs to decide whether a peer can be connected to or if a peer wants you to disconnect. This part is a little confusing because the P2P system doesn't set a lobby ID, but I think the godot's steam networking has/uses lobby IDs for their p2p connections,