Add validation step after metadata ingestion of a metadata-only deposit

Verifying that the metadata that were sent to the ERMDS are effectively the ones you get when calling the get on the same SWHID.

Implementation wise, this reading step is currently done in the deposit test [1] but not in the runtime code [2]. The idea would be to add that reading step in the runtime code.

• [1] https://forge.softwareheritage.org/source/swh-deposit/browse/master/swh/deposit/tests/api/test_deposit_update.py$666-669
• [2] https://forge.softwareheritage.org/source/swh-deposit/browse/master/swh/deposit/api/deposit_update.py$264-278

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Migrated from T2681 (view on Phabricator)