Enable auth for binder-staging

[GeorgianaElena]

While manually deploying the changes in #2277, I didn't realize that we shouldn't touch 2i2c-binder-staging and that this hub doesn't get updates anymore, so I've deployed it 🤦🏼‍♀️ Then, I got sucked into debugging why we didn't manage to get authentication to work in the past and spent too much time on it, to not turn it into a PR, although I realize we want to decommission it? #2026

Developed based on @sgibson91's awesome work in #1579 ❤️

CILogon authentication now works, but I'm not sure about the binder part, as build appears to hang. @2i2c-org/engineering try it out at https://binder-staging.2i2c.cloud/

Todos left:

• Update Binderhub's docs about running an authenticated binderhub https://binderhub.readthedocs.io/en/latest/authentication.html#enabling-authentication
• update cilogon_app.py to support a binderhub key and not just basehub and daskhub
• note somewhere that when creating the cilogon client, you need to provide the jupyterhub url and not the binderhub one
• docs?

[GeorgianaElena]

@sgibson91, I only checked your draft PR after I had some progress with this one, because I had totally forgot about it. Sorry for not building on top of that and opening a separate one :/

Also, looking at it again, it looks like the only thing missing there was a misplaced jupyterhub.hub.config.services instead of jupyterhub.hub.services and a cilogon client created for the binderhub url rather for the hub one pff.

[GeorgianaElena]

Closing in favour of #2393