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.v2/Extensibility/IComposableService.cs

@@ -0,0 +1,21 @@
+//
+// 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.Extensibility
+{
+    /// <summary>
+    /// A Service that expects to lookup other services. Using this interface on an exported service
+    /// will ensure the <see cref="SetServiceProvider(IMultiServiceProvider)"/> method is called during
+    /// service initialization
+    /// </summary>
+    public interface IComposableService
+    {
+        /// <summary>
+        /// Supports settings the service provider being used to initialize the service.
+        /// This is useful to look up other services and use them in your own service.
+        /// </summary>
+        void SetServiceProvider(IMultiServiceProvider provider);
+    }
+}