Change authentication logic to be per-job

[vvmnnnkv]

What?

Currently we authenticate once with model/version supplied in newJob, get worker_id from PyGrid and use it for all jobs.
Instead, we should authenticate for every newJob and store worker_id for given job only.

Why?

Each FL model has own auth settings, hence each job will have individual worker_id that works with corresponding model only.

Breakdown

• Move authentication into newJob
• Save worker_id on Job

Additional Context

See https://github.com/OpenMined/PyGrid/issues/628

[vvmnnnkv]

Let's also rename worker_id to job_id!