cli: add a 'storage' cli group with a 'serve' subcommand
we do add this 'storage' group even if it contains only one subcommand (serve) for UI consistency:
swh storage server
is a more understandable command than just 'swh storage' to start the http server.
We do keep the main swh-storage console_script entry point as is for bw compat, however.
Related to swh-core#1671.
Migrated from D1477 (view on Phabricator)