Add Agent Job and Job Step request handlers (#635)

* Stage changes

* Fix update job request handler

* WIP

* Additional agent handler updates

* Setup agent job step create test

* Fix Step update handler

src/Microsoft.SqlTools.ServiceLayer/DisasterRecovery/BackupOperation/IBackupOperation.cs

@@ -5,6 +5,7 @@
 using Microsoft.SqlServer.Management.Smo;
 using Microsoft.SqlTools.ServiceLayer.Admin;
 using Microsoft.SqlTools.ServiceLayer.DisasterRecovery.Contracts;
+using Microsoft.SqlTools.ServiceLayer.Management;
 using Microsoft.SqlTools.ServiceLayer.TaskServices;
 using System.Data.SqlClient;