Expose optional external id in deposit status endpoint
As this is generated, this could be useful for client who wants to
deposit further new version
From docker-env (manually formatted):
INFO:swh.deposit.cli.deposit:{ 'deposit_id': '3', 'deposit_status': 'done', 'deposit_status_detail': 'The deposit has been successfully loaded into the Software Heritage archive', 'deposit_swh_id': 'swh:1:dir:3b0919ddd42be1ba0405d33f383b6e0ee8dedcba', 'deposit_swh_id_context': 'swh:1:dir:3b0919ddd42be1ba0405d33f383b6e0ee8dedcba;origin=https://forge.softwareheritage.org/source/jesuisgpl/', 'deposit_swh_anchor_id': 'swh:1:rev:7336885eda39fb1f7265c0e8ea75b139cc1773d4', 'deposit_swh_anchor_id_context': 'swh:1:rev:7336885eda39fb1f7265c0e8ea75b139cc1773d4;origin=https://forge.softwareheritage.org/source/jesuisgpl/', 'deposit_external_id': '6a7ef0a1-e88d-4590-b153-2ae9dce3c7ee' # <- HERE we go! }