Allow the deposit protocol to use SWH tags in the deposit namespace

eg. client and external_identifier in requests , deposit_id, deposit_date, deposit_archive, and deposit_status in responses

Currently, we require them to be in the Atom namespace, which they don't belong to.

This is the first step of the resolution of #2758, ie. allow them in the SWH deposit namespace.

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Migrated from T2759 (view on Phabricator)