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

config/crd/bases/actions.summerwind.dev_horizontalrunnerautoscalers.yaml

@@ -129,7 +129,14 @@ spec:
                 description: ScaleTargetRef sis the reference to scaled resource like
                   RunnerDeployment
                 properties:
+                  kind:
+                    description: Kind is the type of resource being referenced
+                    enum:
+                    - RunnerDeployment
+                    - RunnerSet
+                    type: string
                   name:
+                    description: Name is the name of resource being referenced
                     type: string
                 type: object
               scaleUpTriggers: