Add a munge sanity check

[jedel1043]

We should have a method to check if munge is working correctly, since this functionality is used by
all Slurm charms, and we would reduce some code duplication this way.

[NucciTheBoss]

This is something that we could add to mungectl.

Similar to terraform/tofu, we could have a mungectl validate command that emits a 0 exit code if everything is good and then a non-zero exit code if something is wrong with munge such as if NTP is out of sync or there is a key mismatch.