add table designer handlers (#1269)

* add table designer handlers

* add comments

src/Microsoft.SqlTools.ServiceLayer/TableDesigner/Contracts/TableDesignerPropertyIdentifier.cs

@@ -0,0 +1,19 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+namespace Microsoft.SqlTools.ServiceLayer.TableDesigner.Contracts
+{
+    /// <summary>
+    /// The information that can identify a property in a collection.
+    /// </summary>
+    public class TableDesignerPropertyIdentifier
+    {
+        public string ParentProperty { get; set; }
+
+        public int Index { get; set; }
+
+        public string Property { get; set; }
+    }
+}