Add room structure for countdown timers

osu.Game/Online/Multiplayer/Countdown/CountdownChangedEvent.cs

@@ -0,0 +1,22 @@
+// 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
+
+using MessagePack;
+
+namespace osu.Game.Online.Multiplayer.Countdown
+{
+    /// <summary>
+    /// Indicates a change to the <see cref="MultiplayerRoom"/>'s countdown.
+    /// </summary>
+    [MessagePackObject]
+    public class CountdownChangedEvent : MatchServerEvent
+    {
+        /// <summary>
+        /// The new countdown.
+        /// </summary>
+        [Key(0)]
+        public MultiplayerCountdown? Countdown { get; set; }
+    }
+}

osu.Game/Online/Multiplayer/Countdown/MatchStartCountdownRequest.cs

@@ -0,0 +1,23 @@
+// 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 System;
+using MessagePack;
+
+#nullable enable
+
+namespace osu.Game.Online.Multiplayer.Countdown
+{
+    /// <summary>
+    /// A request for a countdown to start the match.
+    /// </summary>
+    [MessagePackObject]
+    public class MatchStartCountdownRequest : MatchUserRequest
+    {
+        /// <summary>
+        /// How long the countdown should last.
+        /// </summary>
+        [Key(0)]
+        public TimeSpan Delay { get; set; }
+    }
+}

osu.Game/Online/Multiplayer/Countdown/StopCountdownRequest.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
+
+using MessagePack;
+
+namespace osu.Game.Online.Multiplayer.Countdown
+{
+    /// <summary>
+    /// Request to stop the current countdown.
+    /// </summary>
+    [MessagePackObject]
+    public class StopCountdownRequest : MatchUserRequest
+    {
+    }
+}