Implements the signevent capability from nostr-protocol/nips#2389: relay admins
can ask the relay to sign an unsigned event template with its own (self) key.
- nip86: SignEvent MethodParams type + "signevent" decode case (round-trips the
{kind, content, tags, created_at} param into a nostr.Event).
- khatru: RelayManagementAPI.SignEvent hook + dispatch returning the signed event
as the result.