Helm chart: add persistence setting for CSI disk

[xeoneox]

Summary

As an administrator I want to set a persistent volume in the helm chart So that I can use a CSI driver with the "local" backend for Restic

Context

I want to mount a volume through CSI and use Restic's local backend. This allows me to use whatever backup medium I can load with a CSI driver, greatly enhancing the flexibility of K8up.

Out of Scope

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Further links

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Acceptance Criteria

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Implementation Ideas

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