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/TaskMetadata.cs

@@ -17,6 +17,11 @@ namespace Microsoft.SqlTools.ServiceLayer.TaskServices
         /// </summary>
         public string Name { get; set; }
 
+        /// <summary>
+        /// Task execution mode (e.g. execute or script)
+        /// </summary>
+        public TaskExecutionMode TaskExecutionMode { get; set; }
+
         /// <summary>
         /// The number of seconds to wait before canceling the task. 
         /// This is a optional field and 0 or negative numbers means no timeout
@@ -41,6 +46,6 @@ namespace Microsoft.SqlTools.ServiceLayer.TaskServices
         /// <summary>
         /// Data required to perform the task
         /// </summary>
-        public object Data { get; set; }
+        public object Data { get; set; }                
     }
 }