Export functions that allow fetching the Open Payments OpenAPI objects in the Open Payments client

[mkurapov]

Context

Since we pack and ship the yaml files directly with the open-payments package, we should have functionality that exports out the OpenAPI objects generated from the Open Payments OpenAPI yaml specs.

• This is primarily to aid with Be able to run Rafiki in Windows Docker rafiki#1562, as this will enable backend auth packages in Rafiki to call functions, and use the OpenAPI validation functions for the Open Payment specs without needing to store the yaml spec files locally.

Todos

• Export functions allowing use of OpenAPI objects directly