Slurm collector: Investigate switch to Slurm REST API

[QuantumDancer]

Instead of retrieving jobs from slurm via the sacct command and subsequent parsing of the output, we could also leverage the SLURM REST API. In particular, the GET /slurmdb/v0.0.39/jobs endpoint looks like it contains most of the functionality that sacct provides.

Plan of action:

1. Investigate if the aforementioned endpoint (or another one) supplies all the functionality needed by us
2. If yes, rework the slurm collector so that it uses the REST API (which should be nicer than parsing the CLI output by hand)