Add AllowExtraDnsLabels to setupKey CRD (#277)

This option was hardcoded to false.
It is now configurable in the SetupKey CRD with a default false value.

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## Summary by CodeRabbit

* **New Features**
* New allowExtraDnsLabels setting for Setup Keys to control whether
peers may include extra DNS labels (disabled by default, immutable after
creation).
* **Documentation**
* API reference updated to document the new allowExtraDnsLabels field
and its default/behavior.

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api/v1alpha1/setupkey_types.go

@@ -16,6 +16,11 @@ type SetupKeySpec struct {
 	// +kubebuilder:validation:XValidation:rule="self == oldSelf",message="ephemeral is immutable"
 	Ephemeral bool `json:"ephemeral"`
 
+	// AllowExtraDnsLabels decides if peers added with the key can have extra DNS labels.
+	// +kubebuilder:default=false
+	// +kubebuilder:validation:XValidation:rule="self == oldSelf",message="allowExtraDnsLabels is immutable"
+	AllowExtraDnsLabels bool `json:"allowExtraDnsLabels"`
+
 	// Duration sets how long the setup key is valid for.
 	// +optional
 	// +kubebuilder:validation:Type=string