feat(kubevirt): add additionalSpec for unmapped spec fields

[Hector295]

The KubeVirt CR spec has many fields. Rather than mapping every field explicitly, this adds an additionalSpec escape hatch that:

1. Allows users to configure any KubeVirt spec field not exposed in values.yaml
2. Uses mergeOverwrite to combine with explicitly mapped fields
3. Gives priority to additionalSpec values when there are conflicts

Example:

$ cat values.yaml 
kubevirt:
  monitorAccount: rancher-monitoring-prometheus
  monitorNamespace: cattle-monitoring-system
  additionalSpec:
    serviceMonitorNamespace: cattle-monitoring-system
    infra:
      nodePlacement:
        tolerations:
          - key: "node-role.kubernetes.io/etcd"
            operator: "Exists"
            effect: "NoExecute"
    configuration:
      developerConfiguration:
        featureGates: ["ExpandDisks"]
        
$ helm template kubevirt assets/kubevirt/kubevirt-0.6.2.tgz -f values.yaml | grep -A22 "# Source: kubevirt/templates/kubevirt.yaml"
# Source: kubevirt/templates/kubevirt.yaml
apiVersion: kubevirt.io/v1
kind: KubeVirt
metadata:
  name: kubevirt
  namespace: default
spec:

  configuration:
    developerConfiguration:
      featureGates:
      - ExpandDisks
  imagePullPolicy: IfNotPresent
  infra:
    nodePlacement:
      tolerations:
      - effect: NoExecute
        key: node-role.kubernetes.io/etcd
        operator: Exists
  monitorAccount: rancher-monitoring-prometheus
  monitorNamespace: cattle-monitoring-system
  serviceMonitorNamespace: cattle-monitoring-system

[Hector295]

Hi @atanasdinov , could you please review this PR when you have a moment?

[Hector295]

@atanasdinov @hardys - just a friendly reminder on this one when you have time. Thanks!