Add additionalVolumeMounts support for flagger chart

[nedal87]

Summary

Add additionalVolumeMounts support to the Flagger Helm chart so users can mount custom files (for example CA bundles or config files) into the Flagger container directly through chart values.

This removes the need for post-render patches (such as Kustomize overlays) for a common customization pattern.

What Changed

• Added additionalVolumeMounts: [] to charts/flagger/values.yaml
• Normalized additionalVolumes default to [] in charts/flagger/values.yaml for type consistency with Kubernetes spec.volumes
• Updated charts/flagger/templates/deployment.yaml:
  • volumes: is rendered when either controlplane.kubeconfig.secretName or additionalVolumes is set
  • volumeMounts: is rendered when either controlplane.kubeconfig.secretName or additionalVolumeMounts is set
  • Existing kubeconfig behavior remains unchanged
• Documented additionalVolumes and additionalVolumeMounts in charts/flagger/README.md

Why

Users already have additionalVolumes, but there is no matching way to mount those volumes into the Flagger container from Helm values alone.

This change closes that gap and makes chart-based customization simpler and more complete while preserving backward compatibility. It also complements PR #1393, which originally added additionalVolumes.

Compatibility

• No change to existing kubeconfig control plane behavior
• Existing installs continue to render and deploy as before
• additionalVolumes now uses a list default ([]), aligned with Kubernetes volumes schema