Extract the `extra_headers` away from `Revision.metadata` into a top-level immutable object

This requires coordinated work on 3 parts:

• swh.model, see !252 (closed)
• swh.storage, see swh-storage!832 (closed)
• swh.loader

See also https://forge.softwareheritage.org/!113

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Migrated from T2423 (view on Phabricator)