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. They should be in the deposit namespace instead.

The migration path would be, in order:

• to allow them in the deposit namespace: T2759
• update the documentation to use the deposit namespace
• deprecate using them in the Atom namespace
• disallow them in the Atom namespace, after everyone stopped using it