Add basic realm models

Only the file related ones are really required outside of tests, but
seems like as good an opportunity as ever to get the rest of the models
into the game project.

osu.Game/Database/IHasRealmFiles.cs

@@ -0,0 +1,18 @@
+// 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.Collections.Generic;
+using osu.Game.Models;
+
+namespace osu.Game.Database
+{
+    /// <summary>
+    /// A model that contains a list of files it is responsible for.
+    /// </summary>
+    public interface IHasRealmFiles
+    {
+        IList<RealmNamedFileUsage> Files { get; }
+
+        string Hash { get; set; }
+    }
+}