Amazon Pay ECE: fix payment method issues for subscriptions

[annemirasol]

Part of #3859
Fixes #3937
Fixes #3938

Changes proposed in this Pull Request:

This PR fixes subscription renewal issues for Amazon Pay, by adding logic to save payment method details in the subscription meta, and creating a payment token for the payment method.

Testing instructions

1. Install Subscriptions, and add a simple subscription product.
2. Turn on the feature flag for Amazon Pay by using the Stripe helper plugin or by setting _wcstripe_feature_amazon_pay to yes in wp_options.
3. Enable Amazon Pay as a payment method.
4. As a shopper, purchase a subscription using Amazon Pay.
5. In My Account > Payment methods, verify that you can see the Amazon Pay saved payment method.
6. In My Account > My Subscription, verify that you can see the subscription's payment info as "Via Amazon Pay (<your Amazon Pay email address>)"
7. Go to the subscription page and process a renewal. Verify that the renewal order succeeds.
8. As a shopper, add a product to cart and go to checkout.
9. Use the Amazon Pay saved payment method to purchase the product.

---

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

Changelog entry

• This Pull Request does not require a changelog entry. (Comment required below)

Changelog Entry Comment

Comment

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)

[malithsen]

Great work here 👏

Tests well and the code looks great. I left a couple of comments mostly out of curiosity and not expecting changes.

[malithsen]

The latest refactor looks great.

✅ Retested subscriptions with Amazon Pay
✅ Smoke tested some flows (Credit card payments, payment method updates)