Add support for merchant validation #646
Description
We need a means to do merchant validation, as per:
https://developer.apple.com/documentation/applepayjs/providing_merchant_validation
However, this is "one and done" event, so it's a good candidate to be a promise. So, sketch:
const url = await PaymentRequest.validationURL;
await merchantSession = await fetch(myServer + url ).then(r => r.text());
const pr = new PaymentRequest({
supportedMethod: "apple-pay",
data: { merchantSession }
});
await pr.show();