Add Damus client tag emission

- Add ClientTagMetadata struct with parsing helpers and documentation
- Append Damus client tags when posting across app, share, and drafts flows
- Gate the behavior behind a new publish_client_tag setting (default on)

Changelog-Added: Add client tag to published events to identify Damus
Ref: https://github.com/damus-io/damus/issues/3323
Signed-off-by: alltheseas <alltheseas@users.noreply.github.com>

highlighter action extension/ActionViewController.swift

@@ -235,7 +235,7 @@ struct ShareExtensionView: View {
             self.highlighter_state = .not_logged_in
             return
         }
-        guard let posted_event = post.to_event(keypair: full_keypair) else {
+        guard let posted_event = post.to_event(keypair: full_keypair, clientTag: state.clientTagComponents) else {
             self.highlighter_state = .failed(error: "Cannot convert post data into a nostr event")
             return
         }