Make binderhub helm chart not depend on jupyterhub

[yuvipanda]

This is a WIP implementation of the proposal that I spoke about with @minrk a while back, and outlined in https://github.com/jupyterhub/binderhub/pull/1862#issuecomment-2197822914.

It basically gets rid of the existing helm chart in this repo which embeds jupyterhub into it, causing all sorts of issues due to the inheritance. We instead want the charts to be composable, so it can be deployed more easily in many different places. This binderhub-service chart was developed thanks to support from GESIS (ty @arnim), and I've merged it in here while preserving all the git history.

I've worked on migrating one of the mybinder.org federation members to using this chart to make sure it works, and 2i2c-bare.mybinder.org now successfully runs on this chart! See https://github.com/jupyterhub/mybinder.org-deploy/pull/3248 for the changes needed

I wanted to work on https://github.com/jupyterhub/binderhub/issues/1940 as well as using kubernetes buildx support for distributed building, and wanted to unify the helm charts before I do so.

TODO

• [x] Rename chart from binderhub-service to binderhub
• [ ] Handle the imageCleaner situation
• [ ] Get all tests to pass
• [ ] Update all the existing deployment documentation to take into account how this should work now
• [ ] Figure out a plan forward for BinderHubSpawnerMixin #jupyterhub > Future of binderhub_mixin.py in binderhub
• [ ] Announce this change, along with the last version of the old chart

[yuvipanda]

Opened a topic to discuss paths forward for the spawner mixin: #jupyterhub > Future of binderhub_mixin.py in binderhub