GET external_subscription by external_id

Added new method to get external_subscription by external_id.

Library/ExternalSubscription.cs

@@ -202,6 +202,20 @@ public static ExternalSubscription Get(string uuid)
 
             return statusCode == HttpStatusCode.NotFound ? null : externalSubscription;
         }
+
+        public static ExternalSubscription GetByExternalId(string externalId)
+        {
+            if (string.IsNullOrWhiteSpace(externalId))
+            {
+                return null;
+            }
+            var externalSubscription = new ExternalSubscription();
+            var statusCode = Client.Instance.PerformRequest(Client.HttpRequestMethod.Get,
+                ExternalSubscription.UrlPrefix + Uri.EscapeDataString("external-id-"+externalId),
+                externalSubscription.ReadXml);
+
+            return statusCode == HttpStatusCode.NotFound ? null : externalSubscription;
+        }
         /// <summary>
         /// Returns a list of external_invoices for this external subscription
         /// </summary>