🌱 kcp: allow unsetting etcd.local, etcd.external and dns

[chrischdi]

What this PR does / why we need it:

All fields for these structs are already mutable. This PR allows unsetting the fields too.

Context: https://kubernetes.slack.com/archives/C8TSNPY4T/p1743103086684469

Note: unsetting one of the following paths using SSA might fail with the following error, when SSA apply uses v1beta1 instead of v1beta2, client side apply works just fine:

• .spec.kubeadmConfigSpec.clusterConfiguration.dns
• .spec.kubeadmConfigSpec.clusterConfiguration.etcd.local

Error example from using ClusterClass and the topology controller hitting this error:

    - lastTransitionTime: "2025-04-10T12:36:00Z"
      message: 'error reconciling the Cluster topology: failed to create patch helper
        for KubeadmControlPlane default/in-memory-10996-jvvxg: server side apply dry-run
        failed for modified object: KubeadmControlPlane.controlplane.cluster.x-k8s.io
        "in-memory-10996-jvvxg" is invalid: spec.kubeadmConfigSpec.clusterConfiguration.etcd:
        Invalid value: "null": spec.kubeadmConfigSpec.clusterConfiguration.etcd in
        body must be of type object: "null"'
      observedGeneration: 4
      reason: ReconcileFailed
      status: "False"
      type: TopologyReconciled```

This is because of dns and etcd structs being optional but not a pointer and kube-apiserver falsely converting it to an invalid state (null).

Workaround: use the latest api version (here v1beta2) in clusterclass.

TODO(chrischdi): open up an issue for the above

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

/area provider/control-plane-kubeadm

[chrischdi]

Note: have to take a look at the following issue when using ClusterClass:

1. Create Cluster with kubeadmControlPlaneTemplate:

spec:
  kubeadmConfigSpec:
    clusterConfiguration:
      ...
      etcd:
        local:
          extraArgs:
            v: "4"
...

2. Create new KubeadmControlPlaneTemplate and refer CC to it and try to unset etcd or etcd.local:

spec:
  kubeadmConfigSpec:
    clusterConfiguration:
      ...
      etcd:
        local: {}
...

Results in the following TopologyReconciled condition:

    - lastTransitionTime: "2025-04-07T11:20:20Z"
      message: 'error reconciling the Cluster topology: failed to create patch helper
        for KubeadmControlPlane default/in-memory-21179-td88v: server side apply dry-run
        failed for modified object: KubeadmControlPlane.controlplane.cluster.x-k8s.io
        "in-memory-21179-td88v" is invalid: spec.kubeadmConfigSpec.clusterConfiguration.etcd:
        Invalid value: "null": spec.kubeadmConfigSpec.clusterConfiguration.etcd in
        body must be of type object: "null"'
      observedGeneration: 4
      reason: ReconcileFailed
      status: "False"
      type: TopologyReconciled

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign neolit123 for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[chrischdi]

/test help

[k8s-ci-robot]

@chrischdi: The specified target(s) for /test were not found.
The following commands are available to trigger required jobs:

/test pull-cluster-api-build-main

/test pull-cluster-api-e2e-blocking-main

/test pull-cluster-api-e2e-conformance-ci-latest-main

/test pull-cluster-api-e2e-conformance-main

/test pull-cluster-api-e2e-latestk8s-main

/test pull-cluster-api-e2e-main

/test pull-cluster-api-e2e-mink8s-main

/test pull-cluster-api-e2e-upgrade-1-32-1-33-main

/test pull-cluster-api-test-main

/test pull-cluster-api-test-mink8s-main

/test pull-cluster-api-verify-main

The following commands are available to trigger optional jobs:

/test pull-cluster-api-apidiff-main

Use /test all to run the following jobs that were automatically triggered:

pull-cluster-api-apidiff-main

pull-cluster-api-build-main

pull-cluster-api-e2e-blocking-main

pull-cluster-api-test-main

pull-cluster-api-verify-main

In response to this:

/test help

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[chrischdi]

/test pull-cluster-api-e2e-conformance-main
/test pull-cluster-api-e2e-latestk8s-main
/test pull-cluster-api-e2e-main
/test pull-cluster-api-e2e-mink8s-main
/test pull-cluster-api-e2e-upgrade-1-32-1-33-main

[chrischdi]

/assign @fabriziopandini