Add a way to get the connection id from a SendStream / RecvStream

[rklaehn]

A SendStream is just a wrapper around a ConnectionRef, so it seems possible to get the connection id. But this is not exposed.

With ConnectionId I mean the stable id of the connection, not the QUIC internal connection id. Basically this thing: https://docs.rs/quinn/latest/quinn/struct.Connection.html#method.stable_id

This would be needed to implement n0-computer/irpc#13

[flub]

(As an aside, I'm still not sure how much I like having issues open on this repo)

What makes you stop at the stable ID? Aren't you really asking to be able to get a Connection struct from a stream?