Add validation of setup key duration (#199)

This adds more validation to the setup key duration to make sure the
unit and format is correct.

examples/refactor/setup-key.yaml

@@ -2,6 +2,7 @@ apiVersion: netbird.io/v1alpha1
 kind: SetupKey
 metadata:
   name: test
-  namespace: netbird
+  namespace: default
 spec:
   name: test
+  ephemeral: true