Support for custom Kubernetes manifests in `KubeConfig`

We are looking at implementing more security features into our DaskHub implementation. One of such features would be deploying a new network policy when creating the cluster through our gateway.

It is currently only possible to modify the pod config, annotations and labels using [`KubeConfig`](https://gateway.dask.org/api-server.html#c.KubeClusterConfig.worker_extra_pod_config), but being able to define extra manifests together with the cluster would be neat :)