Amazon Pay ECE: support payment processing via confirmation tokens

[annemirasol]

Objective
As Amazon Pay does not support manual payment method creation [1], we need to add a new payment processing flow that uses confirmation tokens, instead of manually created payment methods.

Acceptance criteria

• Payment processing flow supports confirmation tokens.*
• No regression for payment processing using manually created payment methods.

Notes

• Other express payment types such as Google Pay should be compatible with confirmation tokens. If the Amazon Pay payment buttons are not yet available, we should be able to hook Google Pay to the flow during development and testing.
• Please document learnings, technical gotchas, and other things of note.

Links

1. https://docs.stripe.com/js/elements_object/create_without_intent#stripe_elements_no_intent-options-paymentMethodCreation
2. https://docs.stripe.com/elements/express-checkout-element/accept-a-payment#create-ct