Support OpenAPI Overlay files

[czechboy0]

Consider supporting OpenAPI Overlay (https://github.com/OAI/Overlay-Specification) files, which would make it easier to manage OpenAPI documents from upstream that need local modifications before passing them to the generator.

Not dissimilar to the support for external references - while both can be handled outside the generator, maybe there's better support we can provide here.

[czechboy0]

Some existing tools for writing and working with overlays:

• https://overlay.speakeasy.com/
• https://github.com/speakeasy-api/openapi-overlay