Add client method for notifying about host changes

osu.Game/Online/RealtimeMultiplayer/ISpectatorClient.cs

@@ -28,6 +28,12 @@ public interface IMultiplayerClient
         /// <param name="user">The user.</param>
         Task UserLeft(MultiplayerRoomUser user);
 
+        /// <summary>
+        /// Signal that the host of the room has changed.
+        /// </summary>
+        /// <param name="userId">The user ID of the new host.</param>
+        Task HostChanged(long userId);
+
         /// <summary>
         /// Signals that the settings for this room have changed.
         /// </summary>