Currently, our API framework is designed in a way that all endpoints (both client-side and server-side) return/accept JSON-like objects (usually encoded with msgpack).
We should abstract this away in the implementation of the framework (in swh-core), so that they never have to dead with dicts representing SWH objects, and use classes from `swh.model.Model` instead.
Protocol-wise, this would not change anything; but it would all endpoints to be typed.