Revert "Remove UmbracoApplicationComponentsInstallingNotification.cs"

This reverts commit f6d39cd7e1.

src/Umbraco.Core/Notifications/UmbracoApplicationComponentsInstallingNotification.cs

@@ -0,0 +1,29 @@
+// Copyright (c) Umbraco.
+// See LICENSE for more details.
+
+using System;
+
+namespace Umbraco.Cms.Core.Notifications
+{
+    // TODO (V10): Remove this class.
+
+    /// <summary>
+    /// Notification that occurs during the Umbraco boot process, before instances of <see cref="IComponent"/> initialize.
+    /// </summary>
+    [Obsolete("This notification was added to the core runtime start-up as a hook for Umbraco Cloud local connection string and database setup. " +
+        "Following re-work they are no longer used (from Deploy 9.2.0)." +
+        "Given they are non-documented and no other use is expected, they can be removed in the next major release")]
+    public class UmbracoApplicationComponentsInstallingNotification : INotification
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UmbracoApplicationComponentsInstallingNotification"/> class.
+        /// </summary>
+        /// <param name="runtimeLevel">The runtime level</param>
+        public UmbracoApplicationComponentsInstallingNotification(RuntimeLevel runtimeLevel) => RuntimeLevel = runtimeLevel;
+
+        /// <summary>
+        /// Gets the runtime level of execution.
+        /// </summary>
+        public RuntimeLevel RuntimeLevel { get; }
+    }
+}