kubermatic-installer: mirror-images should support cilium helm-chart handling (system application)

[renelehmann]

Description of the feature you would like to add / User story

To prepare an air-gapped kkp environment the kubermatic-installer mirror-images command is used to fetch the images or to generate a list of all the needed images.

If e.g. the cilium CNI (as system application) will be used, the tool does not support generating the necessary helm chart images list.
According to the docs https://docs.kubermatic.com/kubermatic/v2.24/tutorials-howtos/networking/cni-cluster-network/
the helm repository can be set and prepared (pull/push) for the local registry.

Alternative approaches

At the moment we will workaround and manually fetch all the supported clilium helm ChartVersion from the related github kkp release (e.g. 2.24.2) and to be able to prepare the local registry with the needed helm charts:
https://github.com/kubermatic/kubermatic/blob/v2.24.2/pkg/cni/cilium/cilium.go

Use cases

Support the preparation, installation and use of an air-gapped kkp environment.

Additional information

It would be nice if some wording to the handling and fetching of the cilium (system application) helm-charts within an air-gapped environment could take place in the offline documentation also: https://docs.kubermatic.com/kubermatic/v2.24/installation/offline-mode/
At the moment it is mentioned on the CNI documentation on the topic "CNI Helm Chart Source" only:
https://docs.kubermatic.com/kubermatic/v2.24/tutorials-howtos/networking/cni-cluster-network/