Enable using preexisting PVC

[lindhe]

Description

This change enables users to provide their own PVC for storing data, rather than forcing them to create a new one. This can be a handy way to decouple the lifecycle management of data from the app, for example to perform backup/restore operations.

Checklist

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[lindhe]

Normally I would recommend following a similar pattern as Bitnami do with existingClaim, especially for charts like this one that depends on Bitnami charts. But turns out that would introduce a lot of extra logic for us compared to this very minimal change, since we already have a name field for the PVC.

I'm pretty sure my current implementation is a non-breaking change.

Looking forward to your input on this!