Creates a billing configuration for a buyer the merchant has onboarded. The buyer must be accepted; a buyer in any other state gets a 422.

Only bank_transfer is available: direct_debit is not available via the API yet.

A second call for the same buyer returns a 422 with the uuid of the existing billing configuration in the error details.

The billing_configuration_uuid from the 201 response can be used for orders with payment_method billing_statement right away, or taken from the billing_configuration/created webhook. If an order comes back with a 422 on billing_configuration_uuid, retry it after the webhook. If the webhook does not arrive, check GET /api/v1/buyers/{buyer_uuid}/billing_configurations.