Add method to get queue's dead letter queue

Fixes #4660

lib/services/serviceBus/lib/servicebusservice.js

@@ -657,6 +657,16 @@ ServiceBusService.prototype.deleteQueue = function (queuePath, callback) {
   this._deleteResource(queuePath, callback);
 };
 
+/**
+* Formats a queue path to point towards its dead letter queue.
+*
+* @param {string}             queuePath                A string object that represents the name of the queue whose dead letter path you want.
+* @return {string} The path to the queue's dead letter queue
+*/
+ServiceBusService.prototype.formatDeadLetterPath = function (queuePath) {
+    return `${queuePath}/$DeadLetterQueue`;
+}
+
 /**
 * Retrieves a queue.
 *