Code generation from OpenAPI description

[judgej]

This will probably be a separate package. The OpenAPI description for the Pi API can be found here:

https://developer.sage.com/api/payments/api/

It will likely not be up-to-date, may not be complete, won't be available in a git repository or anywhere that PRs can be provides or issues raised, but it's a start.

The OpenAPI description is actually spread over at least 13 files (actually more, they are nested), which is a great design choice, but without those files being available in a repository, they are a right royal pain to find and download.

[judgej]

First draft trying to collect the OpenAPI description files together here:

https://github.com/academe/sage-pay-pi-open-api