feat: add until for request filters

This implements an additional filter criteria for selecting events
prior to some timestamp.

See https://github.com/fiatjaf/nostr/issues/39x

src/subscription.rs

@@ -29,6 +29,8 @@ pub struct ReqFilter {
     pub pubkey: Option<String>,
     /// Events published after this time
     pub since: Option<u64>,
+    /// Events published before this time
+    pub until: Option<u64>,
     /// List of author public keys
     pub authors: Option<Vec<String>>,
 }