Add currency restriction pill on Amazon Pay

[hsingyuc]

Fixes #3834

Changes proposed in this Pull Request:

With Amazon Pay being restricted to USD, we need to either hide Amazon Pay in settings if store currency is not USD, or similar to other payment methods, display a "Requires currency" notice beside it. For consistency and future implementation, this PR chose to follow other payment methods, adding a currency restriction pill and rendering next to the payment method in the settings when the shop's currency is not supported.

Testing instructions

1. With the feature flag off -- this is the default -- verify that Amazon Pay is not available in Stripe settings or in any store page.
2. Turn the feature flag on, by setting the _wcstripe_feature_amazon_pay option to yes.
3. Verify that Amazon Pay is now available in Stripe settings.
4. Go to WC->Settings->General->Currency options
5. Set the currency to a non-USD currency
6. Go to WC->Settings->Payments->Stripe->Express checkouts section
7. See Requires currency pill next to Amazon Pay
8. Confirm Amazon Pay button doesn't show on the product, cart, checkout pages
9. Go back to WC->Settings->General->Currency options
10. Set the currency to USD
11. Go back to WC->Settings->Payments->Stripe->Express checkouts section
12. Should not see the Requires currency pill next to Amazon Pay

---

• 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)

[diegocurbelo]

The code changes look good, and it tests well 🚢