Added `useGenericEphemeralVolume` for backup-job

What does this MR do?

This adds a new parameter useGenericEphemeralVolume for the toolbox backup cron job persistence settings.

With this parameter the persistent volume will be create, but only exists as long as the corresponding backup jobs pod (see https://kubernetes.io/docs/concepts/storage/ephemeral-volumes/#generic-ephemeral-volumes). On cloud providers this can drastically reduce the costs for large backup volumes that currently are constantly assigned to the backup jobs.

A new helper function was introduced to not duplicate code for the PVC and the ephemeral volumes volumeClaimTemplate.

NOTICE: Feature state is stable as of K8s 1.23, so this adds a feature-level requirement upon the environment it would be deployed into. (added by @WarheadsSE)

Related issues

Closes #2791 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
  • Yes, except for the Danger Tests which I do not know how to configure correctly...
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
  • Which test plan?
• Documentation created/updated
• Tests added
  • There are no tests for the toolbox and I don´t feel confident writing the first test.
• Integration tests added to GitLab QA
  • I don´t know where to get started there. A pointer on which tests to extend or where to add them would be good...
• Equivalent MR/issue for omnibus-gitlab opened
  • This is not applicable to the omnibus-gitlab project.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.