[Deferred intent] APMs - Process payments with Alipay

[a-danae]

Acceptance criteria

• Payments gets processed as expected (related critical flow: Shopper > Checkout with Alipay) when paying through:
  • The shortcode checkout
  • The block checkout
  • The Pay for order page
• If the payment method type is reusable, it can be saved from:
  • The shortcode checkout
  • The block checkout
  • The Pay for order page
  • The My account -> Payment methods page
• Refunds must work if they're supported
• Errors are handled gracefully:
  • A relevant, user-friendly message must be shown to shoppers
  • A relevant note must be added to the order
  • A detailed message of the error must be logged to the log files if enabled
  • The order status must reflect the payment status

[mattallan]

Deferred intent support for Alipay has been implemented in #2803.

I've confirmed the following is working:

• [x] using Alipay with deferred intents on the shortcode checkout
• [x] using Alipay with deferred intents on the pay for order page
• [x] processing a refund on an alipay purchased with deferred intents
• [x] processing success and failure payments with errors correctly shown on checkout and logged (if enabled)

If the payment method type is reusable, it can be saved from:

Alipay does not support saving the payment method as its payment_fields() function does not render any save payment method checkbox, so I've skipped over these tests.

Closing this one.