Add joined rooms membership endpoint.

src/api.rs

@@ -234,6 +234,12 @@ pub mod r0 {
             join_room_by_id_or_alias
         );
 
+        endpoint!(
+            /// Get a list of the user's current rooms.
+            [r0, membership],
+            joined_rooms
+        );
+
         endpoint!(
             /// Kick a user from a room.
             [r0, membership],