Add callback url to config file

Author: sajjadrad

src/Laravel/config/Zarinpal.php

@@ -21,4 +21,5 @@
     'start_pay_address' =>  env('ZARINPAL_START_PAY_ADDRESS', 'https://www.zarinpal.com/pg/StartPay/'),
     'wsdl_address'      =>  env('ZARINPAL_WSDL_ADDRESS', 'https://www.zarinpal.com/pg/services/WebGate/wsdl'),
     'rest_base_url'     =>  env('ZARINPAL_REST_BASE_URL', 'https://www.zarinpal.com/pg/rest/WebGate/'),
+    'callback_url'      =>  env('ZARINPAL_CALLBACK_URL', 'YOUR-CALLBACK-URL'),
 ];