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.CoreServices/Connection/ReliableConnection/Constants.cs

@@ -0,0 +1,17 @@
+//
+// 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.CoreServices.Connection.ReliableConnection
+{
+    /// <summary>
+    /// Contains common constants used throughout ReliableConnection code.
+    /// </summary>
+    internal static class Constants
+    {
+        internal const int UndefinedErrorCode = 0;
+
+        internal const string Local = "(local)";
+    }
+}