Add HRA support for RunnerSet (#647)

`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
```

Ref #629
Ref #613
Ref #612

api/v1alpha1/horizontalrunnerautoscaler_types.go

@@ -106,6 +106,12 @@ type CapacityReservation struct {
 }
 
 type ScaleTargetRef struct {
+	// Kind is the type of resource being referenced
+	// +optional
+	// +kubebuilder:validation:Enum=RunnerDeployment;RunnerSet
+	Kind string `json:"kind,omitempty"`
+
+	// Name is the name of resource being referenced
 	Name string `json:"name,omitempty"`
 }