feat(iroh-relay): rate limiting of connections to the relay server

[ramfox]

The relay server does currently not have any rate limiting on the number of connections to any of its services (relay/http(s), stun, quic). There should probably be some kind of abuse protection.

Some care needs to be taken however, since by definition the relay is supposed to help NATed clients. So simple per-IP measures are likely to harm legitimate users.

[Arqu]

Is this close-able now with #2961 merged?

[flub]

I don't think so, I think this issue is about the number of connections and connection rate made to the relay server for each service. #2961 is rate-limiting the incoming bandwidth of established connections only.