Allow scaling daskcluster resources directly

The operator allows scaling of workers via `kubectl` by scaling the worker group.

```console
$ kubectl scale --replicas=5 daskworkergroup simple-cluster-default-worker-group
```

If you read the operator docs today this is the only time we mention worker groups as they aren't 100% finished.

Worker groups are an advanced feature and when they do get added to the docs they will probably be further down the page in a more advanced section. I also expect the majority of users to not use them and generally never think about them.

Therefore I think it would be helpful if we could enable scaling on the cluster object itself, which just gets routed through to the default worker group underneath.

```console
$ kubectl scale --replicas=5 daskcluster simple-cluster
```