Add support for remote spec references via $ref

[brittonhayes]

Why?

This would enable users to split up large open api specifications into discrete maintainable parts. Additionally it would allow for the use of remote open api specification schemas to be joined into a local spec.

What?

Ideally someone could pass goapi-gen an open api yaml or json file with a schema that contains $ref attributes and the CLI would resolve those external schemas successfully.

E.g. in a pet store api, put the schema for pet in a pet.yaml folder and be able to import/resolve that with a simple $ref:

Details

getkin/kin-openapi#286

https://github.com/getkin/kin-openapi/blob/17153345908503543b50b7b6409f9d030bae0beb/openapi3/testdata/load_with_go_embed_test.go