Related T821

And not when saving a deposit request.

Related T857

Related T857

This permits to use default configuration when not provided in
configuration file.

This also:
- aligns loader and checker regarding their deposit client use
- improve deposit client's setup regarding authentication and base url
  setup

Related T857

Related T857

Related T857

As we deactivate the checks in tests, all deposit are now passed to
'ready-for-checks' instead of ready.

Related T857

Related T857

Related T857

Summary:
- structured origin_metadata with tool and provider
- used store_metadata function in DepositLoader to add metadata to storage
Note: modification in SWHLoader (loader-core) are in another diff

Reviewers: ardumont

Differential Revision: https://forge.softwareheritage.org/D265

Fix by overriding the parsing done prior to storage in db.  This is
simpler than to use encoder for serializing the right types in the
db (which also needs a superior version of django).