Support HPAScaleToZero using external metric

[nkwangleiGIT]

For some scenarios, we need to scale the pod to zero to save resources, such as GPU or more CPU. And Kubernetes HPA support to scale to zero using feature gate: https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/

HPAScaleToZero: Enables setting minReplicas to 0 for HorizontalPodAutoscaler resources when using custom or external metrics.

So it should be quite easy to integrate with OBI of arbiter, we can achieve this feature.