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[][].
Ensure data present in postgres with extra_headers in the metadata field
are properly supported by the pg-backed storage.
Get rid of the (now useless) git_headers_to_db() converter function.

In Cassandra, store them as frozen<list<list<blob>>>.