Add classes for handling Wpkg message responses

WpkgInstaller/IWpkgMessageCallbackHandlerContract.cs

+using System.ServiceModel;
+
+namespace WpkgInstaller
+{
+    public interface IWpkgMessageCallbackHandlerContract
+    {
+        [OperationContract(IsOneWay = true)]
+        void HandleResponse(WpkgMessageResponse response);
+
+    }
+}

WpkgInstaller/WpkgMessageResponse.cs

+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace WpkgInstaller
+{
+    public class WpkgMessageResponse
+    {
+        public string Message { get; set; }
+
+        public WpkgMessageResponse(string msg)
+        {
+            Message = msg;
+        }
+
+    }
+}