support to add extra arbitrary volumes on MGM, FST

This is required for adding certs to MGM and FST, whether via a secret or copying files out of a PVC (for #74).

It can also be used to mount anything else, e.g. a configmap, cephfs (for #72), etc.

fixes #85 (closed)

Borrowed from https://gitlab.cern.ch/eos/eos-charts/-/tree/fst-sharedfs