model: Add final object_type field on metadata related model objects

To avoid inconsistencies with our other object models

This also fixes current storage build failure

• [1] https://forge.softwareheritage.org/harbormaster/unit/view/744333/

• [2] https://jenkins.softwareheritage.org/job/DSTO/job/tests/1530/console

@vlorentz didn't you open one diff about this (I don't find it)?

Test Plan

tox

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Migrated from D3727 (view on Phabricator)