Add catalogitem update endpoint to public api (#418)

* Initial update endpoint working

* Updated CatalogItem to support more updates; added tests

* Got MediatR domain events working to check for duplicate item names

* Adding reference link

* Remove domain events spike code

* clean up usings

src/ApplicationCore/Exceptions/DuplicateCatalogItemNameException.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace Microsoft.eShopWeb.ApplicationCore.Exceptions
+{
+    public class DuplicateCatalogItemNameException : Exception
+    {
+        public DuplicateCatalogItemNameException(string message, int duplicateItemId) : base(message)
+        {
+            DuplicateItemId = duplicateItemId;
+        }
+
+        public int DuplicateItemId { get; }
+    }
+}