Move unused forked code to external directory (#1192)

* Move unused forked code to external directory

* Fix SLN build errors

* Add back resource provider core since it's referenced by main resource provider project

* Update PackageProjects step of pipeline

external/Microsoft.SqlTools.Hosting.Contracts/Internal/ExitNotification.cs

@@ -0,0 +1,16 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+namespace Microsoft.SqlTools.Hosting.Contracts.Internal
+{
+    /// <summary>
+    /// Defines an event that is sent from the client to notify that
+    /// the client is exiting and the server should as well.
+    /// </summary>
+    public class ExitNotification
+    {
+        public static readonly EventType<object> Type = EventType<object>.Create("exit");
+    }
+}