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/ExportStandardMetadataAttribute.cs

@@ -0,0 +1,44 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Composition;
+
+namespace Microsoft.SqlTools.Hosting.Extensibility
+{
+    /// <summary>
+    /// Base attribute class for all export definitions. 
+    /// </summary>
+    [MetadataAttribute]
+    [AttributeUsage(AttributeTargets.Class)]
+    public abstract class ExportStandardMetadataAttribute : ExportAttribute, IStandardMetadata
+    {
+        /// <summary>
+        /// Base class for DAC extensibility exports
+        /// </summary>
+        protected ExportStandardMetadataAttribute(Type contractType, string id, string displayName = null)
+            : base(contractType)
+        {
+            Id = id;
+            DisplayName = displayName; 
+        }
+        
+
+        /// <summary>
+        /// The version of this extension
+        /// </summary>
+        public string Version { get; set; }
+
+        /// <summary>
+        /// The id of the extension
+        /// </summary>
+        public string Id { get; }
+
+        /// <summary>
+        /// The display name for the extension
+        /// </summary>
+        public virtual string DisplayName { get; }
+    }
+}