Alipay onboarding & supported currencies

[frosso]

Description

Alipay's logic to determine the eligible currencies needs to be duplicated between the backend and the frontend.
It would be great if we could refactor the overall logic to ensure that only the backend's logic can be used and transferred to the frontend, for each payment method.

[frosso]

I spent the points on this ticket looking into possible alternatives.

I documented some of the approaches and problems here: p1739881278173099-slack-CGGCLBN58 and here: #10376

There is ongoing efforts to refactor some of the payment method logic here, which might give a different direction: #10217

I need to write another spike to try some explicit scenarios to document how our code behaves and how Stripe behaves.
I think that what is implemented in our codebase, specifically with the accept_only_domestic_payment check and the get_currencies()/get_countries() is working fine-ish, but should be better refined.

Closing for now, we'll come back to it later.