Tidy up ReloadableObjectFromJson._fetch_inspect_result_json()

[LewisGaul]

The ReloadableObjectFromJson._fetch_inspect_result_json() method implemented for each component is now expected to return an object loaded from JSON, rather than expecting the result from <ctr_exe> <subcmd> inspect to always be a list.

This allows removing the hack in python_on_whales/components/manifest/cli_wrapper.py (and avoids having to do similar for podman pod inspect), which looked like:

    def _fetch_inspect_result_json(self, reference):
        return f'[{run(self.docker_cmd + ["manifest", "inspect", reference])}]'

[gabrieldemarmiesse]

This is a very good change, indeed the previous implementation was hacky.