Openapi codegen interchangeable json formatters

[tg44]

Currently, the code generates case-classes, and imports circe.auto._. Proably we should decouple the case-class from the import/formatter generation (or get the formatter generation as param and use it while we generate the classes), and also add the other supported json libraries one-by-one. We should get the json lib type as an sbt config to make it easily changeable.

[hughsimpson]

Let's close this? We support 3 json libs now, so think it's done in principle