Add extraVolumes support to Praefect

What does this MR do?

Adds the shared gitlab.extraVolumes include to the Praefect StatefulSet's pod-level volumes block.

Praefect already renders extraVolumeMounts, but users could not define the corresponding volumes. This one-line counterpart makes those mounts usable and follows the same helper pattern as the other GitLab subcharts.

Related issues

Closes #5986 (closed)

Verification

• helm template renders a custom gitlab.praefect.extraVolumes entry exactly once in the Praefect StatefulSet.
• git diff --check
• The final diff is one added line in the Praefect StatefulSet template.

Author's checklist

• MR title and description are accurate and descriptive.
• MR targets master.
• The change follows the existing gitlab.extraVolumes helper pattern.
• MR has a green pipeline on GitLab.com.
• MR is labeled workflow::ready for review when the pipeline is green.

Review checklist

• Maintainer review completed.
• Release milestone set.