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.DataProtocol.Contracts/Common/MarkupKind.cs

@@ -0,0 +1,24 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
+
+namespace Microsoft.SqlTools.DataProtocol.Contracts.Common
+{
+    /// <summary>
+    /// Describes the content type that a client supports in various result literals like Hover,
+    /// ParameterInfo, or CompletionItem.
+    /// 
+    /// Please note that MarkupKinds must not start with a '$'. These kinds are reserved for
+    /// internal usage.
+    /// </summary>
+    [JsonConverter(typeof(StringEnumConverter))]
+    public enum MarkupKind
+    {
+        PlainText = 1,
+        Markdown = 2
+    }
+}