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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internal/controller/setupkey_controller.go

@@ -125,7 +125,7 @@ func (r *SetupKeyReconciler) Reconcile(ctx context.Context, req ctrl.Request) (c
 		expiresIn = int(setupKey.Spec.Duration.Seconds())
 	}
 	setupKeyReq := api.PostApiSetupKeysJSONRequestBody{
-		AllowExtraDnsLabels: new(false),
+		AllowExtraDnsLabels: &setupKey.Spec.AllowExtraDnsLabels,
 		AutoGroups:          autoGroupIDs,
 		Ephemeral:           new(setupKey.Spec.Ephemeral),
 		ExpiresIn:           expiresIn,