Review use of Enums

[tcompa]

Close #953

Checklist before merging

• I added an appropriate entry to CHANGELOG.md

[github-actions]

Coverage report

The coverage rate went from 93.1% to 93.08% ⬇️
The branch rate is 84%.

100% of new lines are covered.

Diff Coverage details (click to unfold)

fractal_server/app/api/v1/dataset.py

100% of new lines are covered (95.65% of the complete file).

fractal_server/app/api/v1/project.py

100% of new lines are covered (92.59% of the complete file).

fractal_server/app/models/job.py

100% of new lines are covered (100% of the complete file).

fractal_server/app/runner/init.py

100% of new lines are covered (96.82% of the complete file).

fractal_server/app/schemas/init.py

100% of new lines are covered (100% of the complete file).

fractal_server/app/schemas/applyworkflow.py

100% of new lines are covered (97.1% of the complete file).

[tcompa]

I'm pretty convinced that the bulk of the PR is fine, just removing all Enums from app/models, and only keeping them within app/schemas. I'm left with a doubt about whether to include a validator in ApplyWorkflow - cc @mfranzon