Make WorkerPool implement /scale sub-resource

[juli4n]

The WorkerPool controller owns a deployment that manages all the worker pods, it has a replicas field today, but it's not marked as specReplicasPath. The CRD also doesn't define the scale sub-resource.

This is a prerequisite for any integration with k8s HPA.

[SHRUTI6991]

/assign Shruti Nair (@SHRUTI6991)