Support subdomain_host (CHP needs --host-routing)

[manics]

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The JupyterHub v5 upgrade doc says

All JupyterHub deployments which care about protecting users from each other are encouraged to enable per-user domains, if possible, as this provides the best isolation between user servers.

which sounds like a strong recommendation. This is not currently supported by Z2JH as CHP needs the --host-routing flag. Since CHP is not managed by JupyterHub this isn't automatically configured https://github.com/jupyterhub/jupyterhub/blob/ab43f6beb8571e2b831801089d61144b15951b85/jupyterhub/proxy.py#L738-L739

https://discourse.jupyter.org/t/user-subdomains-oauth-state-missing/29328/4

The main downsides of using hub.config.subdomain_host as the condition instead of a dedicated parameter such as hub.subdomainHost (analogous to hub.baseUrl qhich also requires configuring multiple chart manifests) are it's not part of the schema so won't appear in the generated chart documentation, and it also adds dependencies across manifests with hub.config which I don't think we have, so this increases the maintenance complexity.