implement nip4e interop tags

crates/state/src/nip4e.rs

@@ -5,6 +5,10 @@ use nostr_sdk::prelude::*;
 pub struct Announcement {
     /// The public key of the device that created this announcement.
     public_key: PublicKey,
+    /// The id of the announcement event, for ordering
+    event_id: EventId,
+    /// The creation time of the announcement event
+    created_at: Timestamp,
 }
 
 impl Announcement {
@@ -16,11 +20,20 @@ impl Announcement {
             .and_then(|tag| tag.content())
             .and_then(|c| PublicKey::parse(c).ok())?;
 
-        Some(Self { public_key })
+        Some(Self {
+            public_key,
+            event_id: event.id,
+            created_at: event.created_at,
+        })
     }
 
     /// Returns the public key of the device that created this announcement.
     pub fn public_key(&self) -> PublicKey {
         self.public_key
     }
+
+    pub fn is_newer_than(&self, other: &Self) -> bool {
+        self.created_at > other.created_at
+            || (self.created_at == other.created_at && self.event_id < other.event_id)
+    }
 }