Migrate to FastAPI for autogenerated documentation

[neonjustin]

This mainly applies if you have a running service for the actual REST api.

Probably a nice-to-have feature would be to migrate flask to FastAPI. The two frameworks are very similar and you should be able to make the migration fairly smoothly. The nice thing about FastAPI is that it has auto-generated SwaggerUI and redoc documentation and the framework itself should be able to handle async requests more efficiently than flask.