mariofix / merchants / 33713270081

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providers: validate and remap create_checkout() extra fields per provider

Flow and Khipu accept the same semantic field (customer email) under
different kwarg names (email vs payer_email), and neither validates
presence -- a missing or misnamed kwarg was silently dropped instead
of failing.

Adds checkout_fields/checkout_required class attrs on Provider,
mirroring the existing config_required/config_optional pattern used
for autoload. Provider.__init_subclass__ wraps each subclass's
create_checkout() to validate required fields and remap canonical
names to the provider-specific kwarg name before dispatch, entirely
inside the base class.

No changes to Client, PaymentsResource, cli.py, or any downstream
caller -- client.payments.create_checkout(email=...) now works
uniformly across providers and raises UserError before any network
call if a provider's required field is missing.

Flow's payment/create requires 'email' per their API spec:
https://developers.flow.cl/en/api
Khipu's payer_email remains optional per Payments.create.

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