`HRA.Spec.ScaleTargetRef.Kind` is added to denote that the scale-target is a RunnerSet.
It defaults to `RunnerDeployment` for backward compatibility.
```
apiVersion: actions.summerwind.dev/v1alpha1
kind: HorizontalRunnerAutoscaler
metadata:
name: myhra
spec:
scaleTargetRef:
kind: RunnerSet
name: myrunnerset
```
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