Upstream a way to ensure our DaskClusters shut down if our user server does

[consideRatio]

During the cryo meeting last week I said I thought that if a user server shut down, it would make us delete the user's created dask clusters using two separate mechanism, which the k8s native mechanism would be failsafe.

That was incorrect, but I want to explore if we can implement such feature. I opened dask/dask-gateway#451 regarding that and would be interested in trying to implement it if I conclude its viable and its found to be a wanted feature to add to the project. Some complexity will need to be added, so its absolutely a matter of considering if the complexity is worth it.