Add PayPal support to Payment Element Gateway

Added base PayPal support for Stripe Payment Element with on-session Payment method usage.
Not tested with real payment, but only with test mode for Stripe.

Merged in the latest changes from dev and fixed conflicts, switched to extend StripePaymentMethodTypeBase instead of PaymentMethodTypeBase and opened a merge request so we can patch via composer. Please review!

Thanks for the MR!

There are a couple of items that need some work. I've left comments on the MR.

Thank you for the review! I'll be honest I mostly rerolled what was there, I should've checked the code in more detail to see what it's actually doing, my bad. Will have a look now.

Implemented the changes, please let me know if I missed anything!

sorabh.v6 → made their first commit to this issue’s fork.

@gaydabura why did you make the change you made to src/EventSubscriber/OrderPaymentIntentSubscriber.php?

That change may have sigificant consequences on payment types other than paypal, so it is unlikely this can be committed with it.

@jonathanshaw I added PaymentIntent::STATUS_REQUIRES_ACTION because if the user selects PayPal (or other method which create state "require_action"), the PaymentIntent enters this state. However, if they return without completing the payment and modify their order, we need to update the PaymentIntent amount accordingly.

I understand that this change might affect other payment methods. In this case we might need to provide more flexible logic. If you see any potential issues, let's discuss how to handle them properly.

if they return without completing the payment and modify their order, we need to update the PaymentIntent amount accordingly.

@gaydabura thanks for the reply, much appreciated.

(1) I agree that the scenario you mention is valid, and we need to be sure it addressed.

(2) However, I'm suspect that the fix you suggest is not right. If you look at the examples screenshots in payment intent docs at https://docs.stripe.com/payments/paymentintents/lifecycle it illustrates that at the 'require_action' stage the customer is no longer concerned with verifying the amount and may not even be shown the amount. The change you made in OrderIntentSubscriber might conceivably allow other processes (like staff or price changes) to update the payment intent and change the amount charged but without the customer being aware of the final amount they were paying.

(3) Is there are any reason why this issue of customers returning without completing the payment and modifying their order is a bigger concern with paypal than with card payments?

(4) Given that this concern seems to be equally valid for all payment methods, I believe we should open a new issue for it and address it there for all payment methods. The general experience in the Drupal community seems to be that the more problems we try to solve in a single issue, the less likely we are to solve any of them. Smaller scoped issues make for faster progress.

(5) I wonder if the right solution to the problem you give is in the StripeReview checkout pane. Perhaps if it finds an existing payment intent, it should check the amount of intent still matches the order price.

@jonathanshaw i have removed commit with change. Agree with you, it requires separate issue, with detailed descriptions of problem.

I think this is ready now.