handle add column request (#1272)

* handle add column request

* comments

* rename data to ViewModel

src/Microsoft.SqlTools.ServiceLayer/TableDesigner/Contracts/ViewModel/TableColumnViewModel.cs

@@ -0,0 +1,25 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Newtonsoft.Json;
+
+namespace Microsoft.SqlTools.ServiceLayer.TableDesigner.Contracts
+{
+    /// <summary>
+    /// The view model of a table column object
+    /// </summary>
+    public class TableColumnViewModel : ObjectViewModelBase
+    {
+        public DropdownProperties Type { get; set; } = new DropdownProperties();
+
+        public InputBoxProperties Length { get; set; } = new InputBoxProperties();
+
+        public CheckBoxProperties AllowNulls { get; set; } = new CheckBoxProperties();
+
+        public InputBoxProperties DefaultValue { get; set; } = new InputBoxProperties();
+
+        public CheckBoxProperties IsPrimaryKey { get; set; } = new CheckBoxProperties();
+    }
+}