deposit cli: Open endpoint to validate (technically and basically) metadata submission (offline)

The deposit server currently checks the archive and the metadata. Those checks happen late within the deposit's lifecycle, once client have pushed their deposit and finalized it.

It'd be interesting that the metadata checks could be triggered by the deposit clients prior to actually do any deposits. An offline check which does not involve the deposit server at all. So their feedback loop on their side would be faster.

Let's discuss the need here first ;)

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Migrated from T3360 (view on Phabricator)