Use default storage with existing claim for backend pods (Helm chart)

[sei-jmattson]

Actions before raising this issue

• I searched the existing issues and did not find anything similar.
• I read/searched the docs

Is your feature request related to a problem? Please describe.

Values file override gets cumbersome defining additional volumes for each backend template.

Describe the solution you'd like

When default storage is enabled and an existing claim name is provided, use existing pvc rather than creating one dynamically.

Describe alternatives you've considered

PR to follow.

Additional context

No response