Implement single-room multiplayer room polling

osu.Game/Online/API/Requests/GetRoomRequest.cs

@@ -0,0 +1,19 @@
+// 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.
+
+using osu.Game.Online.Multiplayer;
+
+namespace osu.Game.Online.API.Requests
+{
+    public class GetRoomRequest : APIRequest<Room>
+    {
+        private readonly int roomId;
+
+        public GetRoomRequest(int roomId)
+        {
+            this.roomId = roomId;
+        }
+
+        protected override string Target => $"rooms/{roomId}";
+    }
+}