Allow klipper-helm image version to be overridden

[philip06]

Currently, the latest version of RKE2 forces the usage of rancher/klipper-helm:v0.8.4-build20240523 which when ran against a docker scout command has quite a few critical and high vulnerabilities. With no way of refactoring out or changing the version of the image, it provides a serious problem for hardened environments.

The documentation provides a way to override several images, but many are missing. Adding options for all images, most importantly klipper-helm, would solve this issue entirely. https://docs.rke2.io/reference/server_config#image

The only alternative I've found is to retag a newer version of the image locally, but this is messy and misleading.

ctr -n=k8s.io image pull rancher/klipper-helm:v0.9.1-build20240731

ctr -n=k8s.io image tag rancher/klipper-helm:v0.9.1-build20240731 rancher/klipper-helm:v0.8.4-build20240523

Repost of closed issue: #2160

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