Rewrite indexers as journal clients when relevant

Currently on the metadata indexer was implemented as one but it was dedicated to create one-shot tasks with an indirection on the scheduler.

This would:

• simplify the stack by removing moving parts (scheduler, storage access db for content indexer...).
• allow better monitoring (as we already have a grafana dashboard for journal clients)
• allow indexation to be retried [1] on error
• stop one index computation failure to fail the full batch indexation

Indexers:

• !315 (closed): origin intrinsic metadata
• !353 (closed): origin extrinsic metadata
• !351 (closed): mimetype (content indexer)
• !354 (closed): fossology-indexer (content indexer)

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Migrated from T4273 (view on Phabricator)