Added support for PromptPay on the 2C2P gateway

Spreedly now supports PromptPay payments on the 2C2P gateway for merchants in Thailand using THB. PromptPay purchases use an asynchronous QR flow: customers scan the QR code to pay 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.