refactor: get payment gateways from ERPNext

[s-aga-r]

frappe/erpnext#37182

• GoCardless Settings
• GoCardless Mandate
• GoCardless Templates
• Mpesa Settings
• stripe_integration.py
• Dependencies
• Fix Imports
• Fix Unit Tests

[blaggacao]

Maybe validate_transaction_currency weirdly called by erpnext.accounts.doctype.payment_request.payment_request.{get_request_acount,get_payment_url} and validate_minimum_transaction_amount called by [idem].get_payment_url should both be folded into on_payment_request_submission, instead?

This would reduce the api interface to the necessary minimum and along with frappe/erpnext#37503 would make this refactoring a bit more polished.

However so, at the same time, we need an API expansion in another place (frappe/erpnext#37502) to support known use cases (e.g. Lyra / PayZen) which I'm happy to contribute once stable.

[blaggacao]

Unless this refactoring is barred from orthogonal improvements / changes. According to above comment, I suggest to:

• Standardize around on_payment_request_submission hook for validate_transaction_currency & validate_minimum_transaction_amount (and all other relevant validations)
• Consequentially, throw on payment_gateway_validation instead of casting any errors into silence by default (callers can do so if they so require).
• Amplify the API with on_payment_authorized_redirect as described in the linked PR
• Maybe even reduce the interface around controller.request_for_payment(**payment_record)

If this refactoring is barred from these changes, in order to facilitate a coordinated implementation, I'd suggest to address these issues before the refactoring is completed in order to avoid having to coordinate an API refactoring across repositories later.

[s-aga-r]

@blaggacao do changes separately once this PR is merged.