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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docs/api-reference.md

@@ -315,6 +315,7 @@ _Appears in:_
 | --- | --- | --- | --- |
 | `name` _string_ | Name of the setup key. |  | MinLength: 1 <br /> |
 | `ephemeral` _boolean_ | Ephemeral decides if peers added with the key are ephemeral or not. |  |  |
+| `allowExtraDnsLabels` _boolean_ | AllowExtraDnsLabels decides if peers added with the key can have extra DNS labels. | false |  |
 | `duration` _[Duration](https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.35/#duration-v1-meta)_ | Duration sets how long the setup key is valid for. |  | Pattern: `^([0-9]+(\.[0-9]+)?(m\|h))+$` <br />Type: string <br />Optional: \{\} <br /> |
 | `autoGroups` _[GroupReference](#groupreference) array_ | AutoGroups are groups that will be automatically assigned to peers using setup key. |  | Optional: \{\} <br /> |