Enable CILogon auth for binder-staging

[GeorgianaElena]

#2391 + #1579 (I've probably done it in reverse order, as the merge wasn;t what I expected, but I've fixed it manually)

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 -> [Documentation] Replace references to jupyterhub.auth key with new auth config location jupyterhub/binderhub#1646
• update cilogon_app.py to support a binderhub key and not just basehub and daskhub deployer: Add support for binderhubs in the cilogon_app script #2407
• note somewhere that when creating the cilogon client, you need to provide the jupyterhub url and not the binderhub one
• docs?

[consideRatio]

The changes I see here all makes sense and seem to me like an incremental improvement!

I see some todo notes in the PR descriptions, for example upstream contributions to binderhub docs. I think it makes sense to get this merged and offload todo notes to be tracked a separate issue.