Autoscaling ballista executors in k8s

[edrevo]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Keda is an advanced auto-scaler for k8s will allows deployment to scale to 0 replicas based on custom metrics.

A Ballista scheduler cluster could scale the executor deployment down to 0 replicas when there are no active queries to reduce costs by implementing either the Keda external scaler or the external push scaler.

Describe alternatives you've considered
The HPAScaleToZero feature gate is an alternative route, but it is normally not enabled in k8s deployments and it provides less extensibility than Keda.

Additional context
Original issue: https://github.com/ballista-compute/ballista/issues/616