Configruable scaling cool down period

Implement a way to set a cool-down period for adaptive scaling instead of the hardcoded https://github.com/dask/dask-kubernetes/blob/92714da5785709726f85c4c6ec92451f5c23ad04/dask_kubernetes/operator/controller/controller.py#L816 e.g.

```yaml
apiVersion: kubernetes.dask.org/v1
kind: DaskCluster
metadata:
annotations:
kubernetes.dask.org/cooldown-until-interval: "30s"
name: dask-f3a0c12f
namespace: default
spec:
....
```