feat: add prometheus exporter manager

[NucciTheBoss]

Adds the PrometheusExporterManager class for managing the operations of the Slurm prometheus exporter bundled with the Slurm snap.

PrometheusExporterManager inherits from ServiceManager so that it can enable, disable, and restart the exporter service. The exporter is exposed through the exporter attribute of SlurmManagerBase. This way we can use something like the following block of code to turn on the exporter after we have connected a COS deployment to Charmed HPC:

def _on_integration(self, _: Event) -> None:
    """Handler for when COS is integrated into cluster."""
    # staging operations for setting up observability.
    ...
    self._manager.exporter.enable()

One thing we'll likely need to do in the future is that once we determine how we want to configure the prometheus exporter, we'll need to expose a config interface.

Fixes #19

[jedel1043]

Nice! This will make it really easy to integrate COS into all the charms.

[NucciTheBoss]

Let's go! I'll bump the LIBPATCH version and then merge.