Extract revision's extra_header as a top level attribute
Follows swh.model's evolution for the Revision model class.
In Postgresql, store the extra_headers as a bytea[][].
In Cassandra, store them as msgpack dumped blob.
This needs (unpublished) swh-model!252 (closed) to pass.
Related to swh-model#2423 (closed).
Migrated from D3426 (view on Phabricator)