Add split UPE APMs (eg GiroPay) with deferred intent for Block Checkouts

[james-allan]

Fixes #2790

Changes proposed in this Pull Request:

This PR adds support for all APMs (eg GiroPay) with split UPE and deferred intents.

To achieve this I had to do 2 main things:

1. Turn all our UPE payment method classes into WC_Payment_Gateways. This is required so that all our UPE methods get returned by get_available_payment_gateways() and are registrable as a block checkout payment method with registerPaymentMethod().
2. Making sure that if you select a APM like GiroPay, you are redirected to the URL returned by Stripe in the intent response. This enables the customer to complete the payment at their bank or which ever page is specified by Stripe in the payment intent.

Testing instructions

1. You will need a Stripe account with these APMs enabled.
2. Set your store currency to Euros.
3. Go to WooCommerce > Settings > Payments > Stripe and make sure you have all the APMs enabled.
4. Checkout this branch, add a product to your cart and go to the Block checkout page.
5. Note that all the APMs are now shown.
6. Use each payment method type to make both successful and failed payments.
   • Successful payments
     • Giropay
     • EPS
     • Bancontact
     • iDEAL
     • SEPA (AT611904300234573201) *
     • Przelewy24
   • Failed payments
     • Giropay
     • EPS
     • Bancontact
     • iDEAL
     • SEPA (AT861904300235473202) *
     • Przelewy24
7. After each test make sure:
   1. The order is created while offsite and has a pending status
   2. The order is set to processing or failed when returned to the store
   3. The payment is recorded in your Stripe dashboard appropriately.

Important

* You will need to test SEPA with webhooks enabled as the final status transition (processing or failure) is handled via webhook.

---

• 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)

[james-allan]

Note that the changes on this PR go some way to laying the foundation for APMs on the shortcode checkout too. However none of them appear to work correctly.

eg

I want to take a look into that to see if there's a simple thing I'm missing that will make them work, or whether it will need to be a larger set of changes that should be handled by a separate PR.

[james-allan]

Note that when I changed the WC_Stripe_UPE_Payment_Method class to extend WC_Payment_Gateway I had to make all these properties public to be consistant with that class.

[wjrosa]

It is good to mention that the Stripe account needs to be configured for an EU country here (took me some minutes to find out).

[wjrosa]

Tested all the cases and LGTM! Left a (possible) minor comment

[mattallan]

Hey @james-allan I'm curious if you remember what this change was needed for? Is there a specific gateway that has 'requires_confirmation' as the payment intent status after it has already been confirmed inside create_and_confirm_payment_intent()?

I had to do somethign similar to this change in my PR for Boleto and Oxxo but I'm wondering if there's other gateways that need this.

[james-allan]

When I was working on this PR I needed to change this section so payment methods that need to redirect the user offsite respond to client with the correct redirect URL. Eg this change:

$redirect = $payment_intent->next_action->redirect_to_url->url;

When I was looking into how WooPayments handled this I noticed they had this additional status. I don't recall ever confirming if I received a payment intent status with requires_confirmation though.

You needed it for Boleto and Oxxo though?

[james-allan]

I noticed you're changes in #2823.

I guess whichever one we merge first, we need to condense what that logic needs to look like.

[james-allan]

Ive consolidated that logic in 259ab86