Support OpenAPI Overlay files

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.

No implementation files or tests are named. Start by reading the OpenAPI Overlay Specification and the generator's existing external-reference support, then determine where overlay application would fit before generation. Done means the generator can apply local overlays to upstream OpenAPI documents, with behavior and scope clarified first.

Written by the indexing model from the issue text.