Amazon Pay ECE: add to block cart and checkout

[annemirasol]

Fixes #3732

Changes proposed in this Pull Request:

Adds Amazon Pay express checkout to block cart and block checkout.

Block cart	Block checkout

Testing instructions

1. Turn on the feature flag by setting the _wcstripe_feature_amazon_pay option to yes.
2. In wp-admin Stripe settings, enable Amazon Pay as a payment method.
3. As a shopper, add an item to your cart.
4. Complete the purchase using Amazon Pay, inside the block cart or block checkout page.
5. Verify in wp-admin that the order details are correct, and that the status is Completed or Processing.
   • It may take a while (3-5 minutes) for charges to clear. Status will be Pending payment until then.
6. View block cart or block checkout inside the wp-admin block editor. Verify that the Amazon Pay button is present.
7. Disable Amazon Pay as a payment method, or turn off the feature flag.
8. Verify that Amazon Pay is no longer available inside block cart and checkout.
9. For wp-admin block editor:
   • With the feature flag on and Amazon Pay disabled, the button will still be visible inside the editor. This is a known ECE issue and is documented in Editing the Checkout Block displays express payment method buttons even when they are disabled #3437.
   • With the feature flag off, the Amazon Pay button should no longer be visible inside the editor.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)