Introduce legacy skin configuration

osu.Game/Skinning/LegacySkinConfiguration.cs

@@ -0,0 +1,20 @@
+// 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.Skinning
+{
+    public class LegacySkinConfiguration : DefaultSkinConfiguration
+    {
+        public const double LATEST_VERSION = 2.5;
+
+        /// <summary>
+        /// Legacy version of this skin.
+        /// </summary>
+        public double? LegacyVersion { get; internal set; }
+    }
+
+    public enum LegacySkinConfigurations
+    {
+        Version,
+    }
+}