PayPal: add an extra_data_key_refund parameter to the providers

[radekholy24]

Hello,

in our integration with django-payments, we started to use PayPal refunds. Unfortunately, this has broken our previous expectations on the content of BasePayment.extra_data. Our code expects that payment.extra_data["response"] is the response from the PaypalProvider.process_data call (forever). However, calls to PaypalProvider.refund overwrite this with their response.

This is an attempt to solve this issue by providing a possibility to store refund responses under a different key in payment.extra_data while preserving the original behavior by default for backward compatibility. The new approach was inspired by a similar (not configurable) behavior of PaypalProvider.get_access_token which stores its responses under payment.extra_data["auth_response"].

This is also an attempt to fix the situation upstream although I have no idea how useful this might be for other django-payments users. If it turns out not being usable upstream I'd have to come up with a way to reliably customize PaypalProvider to our needs downstream (considering PaypalProvider has no public API except its constructor and https://django-payments.readthedocs.io/en/latest/backends.html#create-a-provider-class AFAIK).

It is also worth noting that PaypalProvider.refund is not the only method that overwrites payment.extra_data["response"]. But I did not want to overengineer this and since we do not use the other methods, this is enough for us so far. But I can take a look at the rest as well if needed.

I'd be happy for any feedback/opinion. Feel free to request any necessary changes 🙏

[radekholy24]

I've changed my mind. The morning is wiser than the evening. I guess, the right place to fix it is our code. Nevermind. Sorry for the noise.