feat: add support for custom kubeadm image repository

[HoKim98]

What type of PR is this?

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/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

This PR adds support for kubeadm_image_repo variable to utilize custom kubernetes core images, such as kube-apiserver and kube-proxy.

Previously, to achieve this same functionality, we would have to use a variable called kube_image_repo. However, this would have affected the entire ecosystem, including nodelocaldns, scheduler_plugins, and csi_attacher. Instead, it is great for replacing registry.k8s.io, such as for air-gapped deployments.

However, it is not suitable for changing the image repository for Kubernetes itself. And it is unnecessary to change the repository for each image such as kube_apiserver, because these core services are shipped with the Kubernetes version (e.g. v1.32.3). Also, their existence is derived from kubeadm, so manual configuration is unnecessary. Therefore, the core of this PR is to add the ability to replace only the image repository used by kubeadm.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Add support for `kubeadm_image_repo` variable to change kubernetes core image repository (e.g. `kube-apiserver`, `kube-proxy`).

[k8s-ci-robot]

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[tico88612]

I don't understand why need the kubeadm_image_repo variable. I was under the impression that we had offline related operations? (in docs/)

[k8s-ci-robot]

PR needs rebase.

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