register: Include archived_channels in register request

This enables the client to receive metadata about archived streams
in the register response and events, allowing it to handle them properly.

Author: chimnayajith

lib/api/route/events.dart

@@ -18,6 +18,7 @@ Future<InitialSnapshot> registerQueue(ApiConnection connection) {
       'user_avatar_url_field_optional': false, // TODO(#254): turn on
       'stream_typing_notifications': true,
       'user_settings_object': true,
+      'archived_channels': true,
     },
   });
 }