Input struct for a C2B Register URL request.
Required fields: short_code, response_type, confirmation_url, validation_url.
response_type must be either "Completed" or "Cancelled" (sentence case).
This determines what M-PESA does when the validation URL is unreachable:
"Completed"— M-PESA automatically completes the transaction."Cancelled"— M-PESA automatically cancels the transaction.