Update restore service to return default backup folder (#475)

* Fix restore to return default backup folder

* fix break

* fix tests

* fix file validator for invalid filepath

src/Microsoft.SqlTools.ServiceLayer/DisasterRecovery/RestoreOptionsHelper.cs

@@ -53,6 +53,9 @@ namespace Microsoft.SqlTools.ServiceLayer.DisasterRecovery
         //Option name logFileFolder
         internal const string LogFileFolder = "logFileFolder";
 
+        //Option name defaultBackupFolder
+        internal const string DefaultBackupFolder = "defaultBackupFolder";
+
         //The key name to use to set the session id in the request
         internal const string SessionId = "sessionId";