Consolidate the different creation jobs into an organized class/package.

[njbrake]

This issue comes from a comment on a PR about schemas between job and backend.

https://github.com/mozilla-ai/lumigator/pull/692/files/fa86422b1c33d503a786bdc1f8a3e4d840624afd#r1923204888
also
https://github.com/mozilla-ai/lumigator/pull/692/files/fa86422b1c33d503a786bdc1f8a3e4d840624afd#r1923229499

Basically, the different types of Jobs (JobEvalLiteCreate, JobInferenceCreate) could be organized with pydantic descriminators and classes so that when a new job is added, logic to handle the new job would need to be put in as few places as possible.

FYI @javiermtorres since this issue comes out of our discussion about it on the PR.