Introduce a new `swh loader-metadata` subcommand and properly test it
Use the proper entry point to register it in swh.core, and use the root swh cli group in tests to make sure it's properly hooked up.
Staging instance, all changes can be removed at any time
Use the proper entry point to register it in swh.core, and use the root swh cli group in tests to make sure it's properly hooked up.