Refund or cancel a payment

[Refunds](https://docs.adyen.com/api-explorer/#/CheckoutService/latest/post/payments/{paymentPspReference}/refunds) a payment if it has already been captured, and [cancels](https://docs.adyen.com/api-explorer/#/CheckoutService/latest/post/payments/{paymentPspReference}/cancels) a payment if it has not yet been captured. Returns a unique reference for this request. You get the outcome of the request asynchronously, in a [**CANCEL_OR_REFUND** webhook](https://docs.adyen.com/online-payments/reve...