Show file location for dacfx extract and export (#936)

* show file location for dacfx extract and export

* Add TargetDestination field to TaskInfo

* remove setting database and server name to null if there's packageFilePath

* remove unnecessary parameter

src/Microsoft.SqlTools.ServiceLayer/TaskServices/Contracts/TaskInfo.cs

@@ -35,6 +35,11 @@ namespace Microsoft.SqlTools.ServiceLayer.TaskServices.Contracts
         /// </summary>
         public string DatabaseName { get; set; }
 
+        /// <summary>
+        /// Target location for this task
+        /// </summary>
+        public string TargetLocation { get; set; }
+
         /// <summary>
         /// Task name which defines the type of the task (e.g. CreateDatabase, Backup)
         /// </summary>