Add support for OpenAPI overlays

[jamietanna]

This should be useful (and would be likely to link in with the prototype that @deepmap-marcinr has been looking at) to allow

For instance, for the ability to tune the generated output, when you don't have control over the spec

See also: https://docs.bump.sh/guides/openapi/augmenting-generated-openapi/ and https://github.com/pb33f/libopenapi/issues/125