Support confidence on Element

A new field on Element appears 💥

Refs https://gitlab.com/arkindex/requests/-/issues/666

It would be more efficient to store confidence score produced by DLA workers directly on the Element model.

The migration would do 2 things:

• add a nullable confidence field
• copy the existing confidence scores stored as Metadata named confidence, only for elements with a worker version id set

This new field should be available in all ListElements endpoints, but also on RetrieveElement.

Both creation endpoint CreateElement and CreateElements should support the new confidence field as a non-required field, even when worker_version_id is set. It should be fully optional for now.