Add flow for sending match ruleset specific messages to the server

osu.Game/Online/Multiplayer/IMultiplayerRoomServer.cs

@@ -55,6 +55,12 @@ namespace osu.Game.Online.Multiplayer
         /// <param name="newMods">The proposed new mods, excluding any required by the room itself.</param>
         Task ChangeUserMods(IEnumerable<APIMod> newMods);
 
+        /// <summary>
+        /// Send a match ruleset specific request.
+        /// </summary>
+        /// <param name="request">The request to send.</param>
+        Task SendMatchRulesetRequest(MatchRulesetUserRequest request);
+
         /// <summary>
         /// As the host of a room, start the match.
         /// </summary>

osu.Game/Online/Multiplayer/MatchRulesetUserRequest.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.
+
+using System;
+using MessagePack;
+
+namespace osu.Game.Online.Multiplayer
+{
+    /// <summary>
+    /// A request from a user to perform an action specific to the current match ruleset.
+    /// </summary>
+    [Serializable]
+    [MessagePackObject]
+    public abstract class MatchRulesetUserRequest
+    {
+    }
+}

osu.Game/Online/Multiplayer/MatchRulesets/TeamVs/ChangeTeamRequest.cs

@@ -0,0 +1,13 @@
+// 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 MessagePack;
+
+namespace osu.Game.Online.Multiplayer.MatchRulesets.TeamVs
+{
+    public class ChangeTeamRequest : MatchRulesetUserRequest
+    {
+        [Key(0)]
+        public int TeamID { get; set; }
+    }
+}

osu.Game/Online/Multiplayer/MultiplayerClient.cs

@@ -293,6 +293,8 @@ namespace osu.Game.Online.Multiplayer
 
         public abstract Task ChangeUserMods(IEnumerable<APIMod> newMods);
 
+        public abstract Task SendMatchRulesetRequest(MatchRulesetUserRequest request);
+
         public abstract Task StartMatch();
 
         Task IMultiplayerClient.RoomStateChanged(MultiplayerRoomState state)

osu.Game/Online/Multiplayer/OnlineMultiplayerClient.cs

@@ -118,6 +118,14 @@ namespace osu.Game.Online.Multiplayer
             return connection.InvokeAsync(nameof(IMultiplayerServer.ChangeUserMods), newMods);
         }
 
+        public override Task SendMatchRulesetRequest(MatchRulesetUserRequest request)
+        {
+            if (!IsConnected.Value)
+                return Task.CompletedTask;
+
+            return connection.InvokeAsync(nameof(IMultiplayerServer.SendMatchRulesetRequest), request);
+        }
+
         public override Task StartMatch()
         {
             if (!IsConnected.Value)

osu.Game/Tests/Visual/Multiplayer/TestMultiplayerClient.cs

@@ -192,6 +192,8 @@ namespace osu.Game.Tests.Visual.Multiplayer
             return Task.CompletedTask;
         }
 
+        public override Task SendMatchRulesetRequest(MatchRulesetUserRequest request) => Task.CompletedTask;
+
         public override Task StartMatch()
         {
             Debug.Assert(Room != null);