Creates a payment intent. Served by core-api or payments-api depending on the organization's rollout, in the same response shape either way. A request carrying payment_method_options is served by either; payments-api accepts the field without acting on it. A request using setup_future_usage is served by either: its customer_id is validated against customer-api and answers not_found when no such customer exists. An off_session one, a subscription's carrier, is served by core-api until setup intents are served from payments-api, so the carrier and its setup intent live on one service.