Allow scaling CRDs with "scale" subresource

[alshain]

Currently, the Scale interface is only implemented for Deployments and StatefulSets.

It'd be neat if K9S could also scale CRDs that implement the scale subresource.

There should probably be some method isScalable that answers whether a resource provides a scale subresource.

https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#scale-subresource