Implement relay coordination for NWA

[rolznz]

Alby Hub only supports 1 relay. If someone uses a custom relay, they will not be able to connect to their hub through e.g. bitcoin connect which specifies relay.getalby.com as the relay.

Alby Go can use the relay specified in the NWA request, but communicate with Alby Hub using the relay of Alby Go's connection secret. Then it can forward the response to the connecting app.

See nostr-protocol/nips#1818