{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)

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Migrated from D8587 (view on Phabricator)