Support NIP-42 authenticated relay

My relay has a pubkey whitelist, which is problematic because Mostr generates a new keypair for each Fediverse user. The easiest solution would be if I can pass in a private key in the config and whitelist that, since the relay will allow a NIP-42 authenticated user to post stuff on behalf of other pubkeys.

I believe this is possible with RelayPool, but it's undocumented how... https://github.com/adamritter/nostr-relaypool-ts/issues/39