Adding doc and test generation

[noriHanda]

Use case

openapi-generator generates convenient docs and tests, but this package seems to be lacking the feature.

Although being able to generate code with retrofit and freezed, these features are crucial in my team, and therefore we have no choice but to stick with the openapi-generator because having markdown docs accelerates communication between the
frontend and backend team.

Proposal

I would love to see swagger_parser being able to generate docs with or without setting an option.