Register backup to task service for execution (#381)

* Create backup task for execution

* Register backup to task service

* Fix backup task service

* Fix async methods

* Add backup unit test

* Add cancellation token to task service and fix other PR comments

* Add SR and fix other pr comments

* Add comments to methods

* Fixed backup cancel test and casing

* Change sleep time in test

src/Microsoft.SqlTools.ServiceLayer/Localization/sr.strings

@@ -799,5 +799,12 @@ general_containmentType_Partial = Partial
 filegroups_filestreamFiles = FILESTREAM Files
 prototype_file_noApplicableFileGroup = No Applicable Filegroup
 
+############################################################################
+# Backup Service
+Backup_TaskName = Backup Database
 
+############################################################################
+# Task Service
+Task_InProgress = In progress
+Task_Completed = Completed