The existing Job info in the cluster snapshot uses the old definition of Job, which is a single Ray driver (a single `ray.init()` connection).
In the new Job Submission protocol, a Job just specifies an entrypoint which can be any shell command. As such a Job can have zero or multiple Ray drivers. This means we should add a new snapshot entry corresponding to new jobs. We'll leave the old snapshot in place for legacy jobs.
- Also fixes `get_all_jobs` by using the appropriate KV namespace, and stripping the job key KV prefix from the job ID. It wasn't working before.
- This PR also unifies the datatype used by the GET jobs/ endpoint to be the same as the one used by the new jobs cluster snapshot. For backwards compatibility, the `status` and `message` fields are preserved.
