Announce `civkitservice` registration to clients

Current `civkitservice` API enable one to host a civkit service on top of a blessed relay (`RegisterRequest`). On service registration the service policy should be propagated to relay peers and clients. Service might say what is actually offered (e.g board type) and for how long the service is accessible on this relay (e.g X days, Y months or indefinite). Service registration might happen in the future over Nostr websocket for more interface flexibility.