Add simple user state class

osu.Game/Online/RealtimeMultiplayer/MultiplayerClientState.cs

@@ -0,0 +1,15 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Online.RealtimeMultiplayer
+{
+    public class MultiplayerClientState
+    {
+        public MultiplayerClientState(in long roomId)
+        {
+            CurrentRoomID = roomId;
+        }
+
+        public long CurrentRoomID { get; }
+    }
+}