add table designer handlers (#1269)

* add table designer handlers

* add comments

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

@@ -0,0 +1,34 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System.Runtime.Serialization;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
+
+namespace Microsoft.SqlTools.ServiceLayer.TableDesigner.Contracts
+{
+    [JsonConverter(typeof(StringEnumConverter))]
+    public enum DesignerEditType
+    {
+        [EnumMember(Value = "Add")]
+        Add,
+        [EnumMember(Value = "Remove")]
+        Remove,
+        [EnumMember(Value = "Update")]
+        Update,
+    }
+
+    /// <summary>
+    /// The information about a change made inside the table designer.
+    /// </summary>
+    public class TableDesignerChangeInfo
+    {
+        public DesignerEditType Type { get; set; }
+
+        public object Property { get; set; }
+
+        public object Value { get; set; }
+    }
+}