Update all EF based models to implement new read only interfaces

osu.Game/Database/IHasOnlineID.cs

@@ -0,0 +1,15 @@
+// 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
+
+namespace osu.Game.Database
+{
+    public interface IHasOnlineID
+    {
+        /// <summary>
+        /// The server-side ID representing this instance, if one exists.
+        /// </summary>
+        int? OnlineID { get; }
+    }
+}