Add update manager which performs no update action

This is to be used in cases where updates are handled by an external
means. See
https://github.com/flathub/flathub/pull/2619#issuecomment-969731305 for
initial usage.

osu.Game/Updater/GitHubRelease.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.
+
+using System.Collections.Generic;
+using Newtonsoft.Json;
+
+namespace osu.Game.Updater
+{
+    public class GitHubRelease
+    {
+        [JsonProperty("html_url")]
+        public string HtmlUrl { get; set; }
+
+        [JsonProperty("tag_name")]
+        public string TagName { get; set; }
+
+        [JsonProperty("assets")]
+        public List<GitHubAsset> Assets { get; set; }
+    }
+}