Add a free-form identifier for payment request. This will be set using PaymentDetailsInit and reflected through PaymentRequest and PaymentResponse and to payment apps. If no identifier is specified a unique identifier will be created.
Docs: https://github.com/w3c/browser-payment-api/issues/388 https://github.com/w3c/browser-payment-api/pull/426 https://developer.mozilla.org/en-US/docs/Web/API/PaymentRequest/PaymentRequest https://developer.mozilla.org/en-US/docs/Web/API/PaymentRequest https://developer.mozilla.org/en-US/docs/Web/API/PaymentResponse