feat(NIP-42): pubkey authentication

Configurable in `config.toml`.  Limited functionality, but this does
send metadata to gRPC for event authorization.

fixes: https://todo.sr.ht/~gheartsfield/nostr-rs-relay/66

src/notice.rs

@@ -16,6 +16,7 @@ pub struct EventResult {
 pub enum Notice {
     Message(String),
     EventResult(EventResult),
+    AuthChallenge(String)
 }
 
 impl EventResultStatus {