{Content|Directory}Loader: Adapt support for checksums
This moves away the loaders from the 'integrity' fields which is now dealt with lister side. Instead the lister is send a dictionary of hex hashes.
This also improves the loader to check those checksums when retrieving the artifact (content or tarball). Thanks to a bump in the swh.model version, this is now able to deal with sha512 checksums checks as well.
This echoes with the ongoing work for the package loaders [1].
Related to T3781 Depends on !436 (closed)
- [1] !326 (closed)
Migrated from D8587 (view on Phabricator)