feat(stacked-PR): create cluster with custom helm chart config

[supershal]

What problem does this PR solve?:

• Stacked on feat: API for custom helm chart configuration #1015
• Implements API defined in feat: API for custom helm chart configuration #1015
• Modified HelmChartGetter to first search chart info in the ConfigMap provided with the cluster. If the configMap does not contain component key for an Addon's chart information then fall back to default-helm-addons-config that is shipped with CAREN.
• Preserves existing behavior where addons are installed using default-helm-addons-config configmap
• A webhook to ensure that the referenced ConfigMap exists.

How Has This Been Tested?:

[X] unit test for addon webhook
[]unit test for HelmChartGetter (will add new commit)
[]e2e tests with custom configmap (Will file stacked PR)
[X] manual test on Nutanix Cluster

• Custom Configmap

apiVersion: v1
data:
  nutanix-ccm: |
    ChartName: nutanix-cloud-provider
    ChartVersion: 0.5.0
    RepositoryURL: 'https://nutanix.github.io/helm/'
kind: ConfigMap
metadata:
  name: shalin-upgrade-addons-helm-addons-config
  namespace: default

• Cluster with variable

variables:
    - name: clusterConfig
      value:
        addons:
          helmChartConfig:
            configMapRef:
              name: shalin-upgrade-addons-helm-addons-config
...
...

• HCP for Nutanix CCM using chart info from the custom configmap:

apiVersion: addons.cluster.x-k8s.io/v1alpha1
kind: HelmChartProxy
metadata:
  generation: 1
  name: nutanix-ccm-01949566-8814-7427-bec9-eecdfae20c0a
  namespace: default
  ...
spec:
  chartName: nutanix-cloud-provider
  releaseName: nutanix-ccm
  repoURL: https://nutanix.github.io/helm/
  ...
  version: 0.5.0
status:
  conditions:
  - lastTransitionTime: "2025-01-23T23:03:32Z"
    status: "True"
    type: Ready
  ...
  matchingClusters:
  - apiVersion: cluster.x-k8s.io/v1beta1
    kind: Cluster
    name: shalin-upgrade-addons
    namespace: default
  observedGeneration: 1 

• HCP for addon (ex. Cilium) NOT referenced in the custom configmap. It uses info from default helm chart configmap

apiVersion: addons.cluster.x-k8s.io/v1alpha1
kind: HelmChartProxy
metadata:
  generation: 1
  name: cilium-01949566-8814-7427-bec9-eecdfae20c0a
  namespace: default
  ...
spec:
  chartName: cilium
  releaseName: cilium
  repoURL: oci://helm-repository.default.svc/charts
  ...
  version: 1.16.4
status:
  conditions:
  - lastTransitionTime: "2025-01-23T23:06:12Z"
    status: "True"
    type: Ready
  ...
  matchingClusters:
  - apiVersion: cluster.x-k8s.io/v1beta1
    kind: Cluster
    name: shalin-upgrade-addons
    namespace: default
  observedGeneration: 1

Special notes for your reviewer:

The discussion for the API is still going on at #1015
While main logic would remain same, if any changes proposed in the API, I will update this PR with the implementation.

Open question:
Should we set owner reference in the referenced configmap?