API Updates

[dcr-stripe]

Codegen for openapi v184.
r? @pakrym-stripe
cc @stripe/api-libraries

Changelog

• Change type of Treasury.OutboundTransfer.destination_payment_method from string to string | null
• Change the return type of CustomerService.fundCashBalance test helper from CustomerBalanceTransaction to CustomerCashBalanceTransaction.
  • This would generally be considered a breaking change, but we've worked with all existing users to migrate and are comfortable releasing this as a minor as it is solely a test helper method. This was essentially broken prior to this change.