Added support for Vietnamese Bank Transfer on the 2C2P gateway

Spreedly now supports Vietnamese Bank Transfer payments on the 2C2P gateway for merchants in Vietnam using VND. Vietnamese Bank Transfer purchases use an asynchronous offsite flow: customers are redirected to their bank's payment page to authorize payment while the transaction remains pending until 2C2P confirms the final status through callback and payment inquiry. For more information, please see the 2C2P gateway guide.