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/utils.rs

@@ -1,6 +1,7 @@
 //! Common utility functions
 use bech32::FromBase32;
 use std::time::SystemTime;
+use url::Url;
 
 /// Seconds since 1970.
 #[must_use] pub fn unix_time() -> u64 {
@@ -33,6 +34,10 @@ pub fn nip19_to_hex(s: &str) -> Result<String, bech32::Error> {
     })
 }
 
+pub fn host_str(url: &String) -> Option<String> {
+    Url::parse(url).ok().and_then(|u| u.host_str().map(|s| s.to_string()))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;