Fix backup service to have multiple backup instances (#396)

* Allow multiple backups per backup service instance

* Add test to run multiple backups

* Update backup cancelTask method signature

* Fix to have multiple backup instances and add more tests

* Address PR comments

* Remove double new lines

* Add Azure check for backup operations

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

@@ -37,5 +37,10 @@ namespace Microsoft.SqlTools.ServiceLayer.TaskServices
         /// Database name this task is created for
         /// </summary>
         public string DatabaseName { get; set; }
+
+        /// <summary>
+        /// Data required to perform the task
+        /// </summary>
+        public object Data { get; set; }
     }
 }