Integrate generate script with task service (#426)

* Support generate script for backup

* change

* update task service data contract for Generate Script

* more changes

* update test

* add comments

* Add missing files

* update stub backup operation for testing

* pr comments

* remove empty space

* Fix tests

* Add unit/integration tests and isCancelable to TaskInfo

* address pr comments

* pr comments - fix tests

* fix minor issue

* fix minor issues

* remove unused variable

src/Microsoft.SqlTools.ServiceLayer/TaskServices/TaskExecutionMode.cs

@@ -0,0 +1,29 @@
+//
+// 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.TaskServices
+{
+    /// <summary>
+    /// Task execution mode
+    /// </summary>
+    public enum TaskExecutionMode
+    {
+        /// <summary>
+        /// Execute task
+        /// </summary>
+        Execute,
+
+        /// <summary>
+        /// Script task
+        /// </summary>
+        Script,
+
+        /// <summary>
+        /// Execute and script task
+        /// Needed for tasks that will show the script when execution completes
+        /// </summary>
+        ExecuteAndScript
+    }
+}