feat: add support to generate openapi / swagger spec

[dgaedcke]

Description

It's incredibly convenient and powerful to be able to annotate server endpoints with the receive/return types (eg data classes) and then to use those specs to generate language specific code for any clients of the server.

Requirements

Generate the spec in v3 JSON format https://swagger.io/docs/specification/about/

https://pub.dev/packages/openapi_generator_annotations

https://www.dhiwise.com/post/how-openapi-generator-can-streamline-the-flutter-development

[dgaedcke]

ah sorry ... I found that you have an open feature request over here: https://github.com/VeryGoodOpenSource/dart_frog/issues/152