Allow extra volumes for clickhouse-backup

[SuperTux88]

Description

When using GCS as upload target it requires a client-key JSON file, which needs to be mounted as volume.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How has this been tested?

Deployment in google cloud.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works