Add stateful client interface

osu.Game/Online/RealtimeMultiplayer/IStatefulMultiplayerClient.cs

@@ -0,0 +1,17 @@
+// 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.
+
+#nullable enable
+
+namespace osu.Game.Online.RealtimeMultiplayer
+{
+    /// <summary>
+    /// A multiplayer client which maintains local room and user state. Also provides a proxy to access the <see cref="IMultiplayerServer"/>.
+    /// </summary>
+    public interface IStatefulMultiplayerClient : IMultiplayerClient, IMultiplayerServer
+    {
+        MultiplayerUserState State { get; }
+
+        MultiplayerRoom? Room { get; }
+    }
+}