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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config/crd/bases/netbird.io_setupkeys.yaml

@@ -46,6 +46,14 @@ spec:
           spec:
             description: SetupKeySpec defines the desired state of SetupKey.
             properties:
+              allowExtraDnsLabels:
+                default: false
+                description: AllowExtraDnsLabels decides if peers added with the key
+                  can have extra DNS labels.
+                type: boolean
+                x-kubernetes-validations:
+                - message: allowExtraDnsLabels is immutable
+                  rule: self == oldSelf
               autoGroups:
                 description: AutoGroups are groups that will be automatically assigned
                   to peers using setup key.
@@ -96,6 +104,7 @@ spec:
                 minLength: 1
                 type: string
             required:
+            - allowExtraDnsLabels
             - ephemeral
             - name
             type: object