refund method
Refund a previous payment
amount - Refund amount in cents (null for full refund)
paymentReferenceId - Reference ID of the payment to refund
refundReference - Unique reference ID for this refund
Implementation
Future<RefundResult> refund({
int? amount,
required String paymentReferenceId,
required String refundReference,
}) {
throw UnimplementedError('refund() has not been implemented.');
}