feat: add missing export for on_activity_added state

Author: xsahil03x

packages/stream_feeds/lib/src/state.dart

@@ -16,6 +16,7 @@ export 'state/comment_reaction_list.dart';
 export 'state/comment_reaction_list_state.dart';
 export 'state/comment_reply_list.dart';
 export 'state/comment_reply_list_state.dart';
+export 'state/event/on_activity_added.dart';
 export 'state/feed.dart';
 export 'state/feed_list.dart';
 export 'state/feed_list_state.dart';