Payments API v2

Enums grow without a new version: statuses, status_details.code, next_action.type, event types, method_type, error codes. Treat an unknown status as not final, an unknown status_details.code as provider_declined, an unknown next_action.type as a reason to stop and ask, an unknown event type as a reason to re-read the transaction, an unknown error code by its HTTP status.