Amazon Pay ECE: add payment method class

[annemirasol]

Part of #3859

Changes proposed in this Pull Request:

As part of supporting subscriptions and payment tokens for Amazon Pay, this PR introduces Amazon Pay's own payment method class, i.e. WC_Stripe_UPE_Payment_Method_Amazon_Pay.

Succeeding PRs will:

• update Amazon Pay setting controls to to turn the feature on and off by adding and removing it as an enabled payment method, similar to how we do Link
• update the Amazon Pay express checkout element to use amazon_pay (instead of stripe) as the payment method
• add code for supporting automatic subscription renewals, which include creating payment tokens for Amazon Pay(i.e. saved payment methods).

Testing instructions

The new payment method will be used in succeeding PRs. This PR should not introduce any changes in behavior.

1. Keep an eye on the browser console and error logs while testing.
2. Smoke test possible regressions by:
   • purchasing a simple product using any non-Amazon Pay method.
   • purchasing a simple product using Amazon Pay.
   • visiting the Stripe Payment Methods page in wp-admin, and making sure everything loads correctly.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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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)

[hsingyuc]

@annemirasol, have you seen this error before? I can't checkout with Amazon Pay.

[annemirasol]

@annemirasol, have you seen this error before? I can't checkout with Amazon Pay.

@hsingyuc I talked to Stripe support on Discord yesterday, because Amazon Pay was missing despite being enabled, and they told me they were having issues with Amazon Pay not working properly yesterday, so they took it down temporarily. This might be one of the issues.

Anyway, I checked just now and everything seems to be working fine!

[hsingyuc]

@annemirasol, good work!
✅ No console error
✅ Smoke tests are as expected