loader.cli: Add 2 subcommands, one to load, one to introspect
2 subcommands:
- one to load:
swh loader run <type> <url> <options>
- one to list supported loader or instrospect the loader's extra arguments (most of the time url should be enough but not always)
swh loader list
swh loader list <type>
Related to T2134
Test Plan
tox
Migrated from D2430 (view on Phabricator)