Update PRB payment method generation to use PaymentMethods rather than sources

[james-allan]

Fixes #3506

Changes proposed in this Pull Request:

• Updates the PRB JS code so it creates a PaymentMethod object (pm_) rather than a source src_.
• Update the PRB flow so it goes via the new deferred intent flow rather than legacy payment processing.

Testing instructions

GENERAL TESTING

1. Make sure the ECE feature flag isn't set.
2. Enable the express payment buttons.
3. Complete purchases using PRBs on the product, cart and checkout pages.
4. Confirm that payments are completed as expected.

SUBSCRIPTIONS FREE TRIAL

1. Create a virtual subscription product with a free trial (screenshot)
2. Purchase the product using a PRB.
3. On develop there are 4 things to notice.
   1. The resulting payment method will be a source (src_).
   2. There will be set up intent request with a failure with the following error message.
   3. The src_ won't be attached to the customer.
   4. There is no payment method token saved in the WooCommerce store. Note that the source when it was created is reusable and in my testing it does work for future renewal payments.
4. On this branch the resulting payment method will be a pm_ and will be attached to the customer.
5. Because the PM is attached to the customer, there is a saved token.

Before	After

7. Process a renewal. Note that it is successful.

• [ ] Covered with tests (or have a good reason not to test in description ☝️)
• [x] 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)

[annemirasol]

Nice work, James ✨ Was able to reproduce the issue, and verify that the PR fixes it.

❓ When Legacy Checkout is enabled, and I try to use PRB on checkout, I get this generic error: Payment processing failed. Please retry.. Is that because we're removing the source event handler for paymentRequest? Should we be hiding PRBs if Legacy is enabled, then?

[james-allan]

When Legacy Checkout is enabled, and I try to use PRB on checkout

Oh good catch. Legacy Checkout wasn't something I had considered. Let me take a look at whether I can still honor the legacy checkout request format.

[james-allan]

Ok thanks again for the review @annemirasol. I've pushed b970ed4 which brings back the stripe_source key which the legacy checkout's process_payment() function expects/requires to process the payment successfully.

There's a little bit of redundancy in this approach passing both stripe_source and wc-stripe-payment-method with the same pm_ ID just so both checkout integrations have their expected args. I'm curious if you think there's a better approach. :)

[annemirasol]

There's a little bit of redundancy in this approach passing both stripe_source and wc-stripe-payment-method with the same pm_ ID just so both checkout integrations have their expected args. I'm curious if you think there's a better approach. :)

I suppose we could add some logic to decide which arg to pass depending on which checkout mode is enabled, but I'm okay with just passing both. I like this compact approach, and we can clean up later when we drop support for legacy checkout.