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/Error.cs

@@ -0,0 +1,23 @@
+//
+// 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 the message contract for errors returned via <see cref="Microsoft.SqlTools.Hosting.Protocol.RequestContext.SendError"/>
+    /// </summary>
+    public class Error
+    {
+        /// <summary>
+        /// Error code. If omitted will default to 0
+        /// </summary>
+        public int Code { get; set; }
+
+        /// <summary>
+        /// Error message
+        /// </summary>
+        public string Message { get; set; }
+    }
+}