PaymentMethodConfig indicating what fields it needs for billing (possibly validation?)

[nerrad]

This issue is initially for consideration/discussion as an alternative to validation proposed as a part of onCheckoutDataProcessing event in #1939 .

For this alternative, Checkout would still be responsible for rendering billing data fields (fields not specific to payment method like cc, or cc expiry etc), but payment methods would indicate what fields are required as a part of their registration process.

So for example, they might register something like this as a part of the configuration:

const config = {
 ...existingConfig,
billingFieldsRequired: [ 'country', 'state', 'postalCode' ],
};

Then checkout can use this as a signal for setting/validating those fields as required when processing and that payment method is selected/active.

We might still want to allow for payment methods to do additional validation (as per #1939) but this can put the billing field validation into the checkout.

[nerrad]

This is no longer applicable so closing.