Add Science Platform support for multiple user session mounts and types

Support, in a general manner, a way to allow helm deployments to specify what mounts become available in user sessions (their containers).  Currently this is essentially hard-coded to mount the cavern user storage and scratch space.  But other deployments have shown interest in mounting other things, such as CVMFS (PR here:  https://github.com/opencadc/science-platform/pull/645), and perhaps Rucio Storage to get at SKA data.  The work would be to generalize the mounting and give deployers options on the number and types of mount, as well as integrate that CVMFS PR into skaha in a way that makes the most sense.