Support autoscaling #3
Description
As the service.Spec describes, we have minReplicas and maxReplicas, what we hope to do is adjust the number based on the traffic, aka. servreless. We can use ray or keda/knative as alternatives, but here we hope we can have a simple implementation, then no need to depend on other libraries.
For the first step, let's integrate with HPA for autoscaling capacities.