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/Utilities/Union.cs

@@ -0,0 +1,103 @@
+//
+// 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 Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+
+namespace Microsoft.SqlTools.DataProtocol.Contracts.Utilities
+{
+    internal interface IUnion
+    {
+        object Value { get; set; }
+    }
+
+    [JsonConverter(typeof(UnionJsonSerializer))]
+    public class Union<T1, T2> : IUnion
+    {
+        protected T1 value1;
+        protected T2 value2;
+
+        internal Union() {}
+        
+        public object Value
+        {
+            get => (object) value1 ?? value2;
+            set
+            {
+                if (value == null)
+                {
+                    throw new ArgumentException();
+                }
+                
+                if (value is T1)
+                {
+                    value1 = (T1) value;
+                } 
+                else if (value is T2)
+                {
+                    value2 = (T2) value;
+                }
+                else
+                {
+                    throw new InvalidOperationException();
+                }
+            }
+        }
+    }
+
+    public class UnionJsonSerializer : JsonConverter
+    {
+        public override bool CanRead => true;
+        public override bool CanWrite => true;
+
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            writer.WriteValue(((IUnion)value).Value);
+        }
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        {
+            // Check for null first
+            JToken jToken = JToken.Load(reader);
+            if (jToken.Type == JTokenType.Null)
+            {
+                return null;
+            }
+            
+            // Cast to an object
+            JObject jObject = (JObject)jToken;
+
+            // Try to convert to 
+            object value = null;
+            foreach (Type t in objectType.GenericTypeArguments)
+            {
+                try
+                {
+                    value = jObject.ToObject(t);
+                    break;
+                }
+                catch (Exception)
+                {
+                    // Ignore any failure to cast, we'll throw if we exhaust all the conversion options
+                }
+            }
+
+            if (value == null)
+            {
+                throw new InvalidCastException();
+            }
+            
+            IUnion result = (IUnion)Activator.CreateInstance(objectType);
+            result.Value = value;
+            return result;
+        }
+
+        public override bool CanConvert(Type objectType)
+        {
+            return objectType.IsAssignableFrom(typeof(IUnion));
+        }
+    }
+}