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/NoActionUpdateManager.cs

@@ -0,0 +1,67 @@
+// 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.Linq;
+using System.Threading.Tasks;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics.Sprites;
+using osu.Framework.Platform;
+using osu.Game.Online.API;
+using osu.Game.Overlays.Notifications;
+
+namespace osu.Game.Updater
+{
+    /// <summary>
+    /// An update manager that shows notifications if a newer release is detected.
+    /// This is a case where updates are handled externally by a package manager or other means, so no action is performed on clicking the notification.
+    /// </summary>
+    public class NoActionUpdateManager : UpdateManager
+    {
+        private string version;
+
+        [Resolved]
+        private GameHost host { get; set; }
+
+        [BackgroundDependencyLoader]
+        private void load(OsuGameBase game)
+        {
+            version = game.Version;
+        }
+
+        protected override async Task<bool> PerformUpdateCheck()
+        {
+            try
+            {
+                var releases = new OsuJsonWebRequest<GitHubRelease>("https://api.github.com/repos/ppy/osu/releases/latest");
+
+                await releases.PerformAsync().ConfigureAwait(false);
+
+                var latest = releases.ResponseObject;
+
+                // avoid any discrepancies due to build suffixes for now.
+                // eventually we will want to support release streams and consider these.
+                version = version.Split('-').First();
+                string latestTagName = latest.TagName.Split('-').First();
+
+                if (latestTagName != version)
+                {
+                    Notifications.Post(new SimpleNotification
+                    {
+                        Text = $"A newer release of osu! has been found ({version} → {latestTagName}).\n\n"
+                               + "Check with your package manager / provider to bring osu! up-to-date!",
+                        Icon = FontAwesome.Solid.Upload,
+                    });
+
+                    return true;
+                }
+            }
+            catch
+            {
+                // we shouldn't crash on a web failure. or any failure for the matter.
+                return true;
+            }
+
+            return false;
+        }
+    }
+}