Allow setting extraContainers, extraVolumes, etc as dicts

[yuvipanda]

Proposed change

Right now, extraContainers, extraVolumes, extraVolumeMounts, etc are lists. This matches what the k8s API needs. However, this makes overriding them in helm config very difficult, as there's no way to spend to a list or modify individual elements.

For example, if you have this config in your generic-to-all-hubs helm config:

hub:
  initContainers:
      - name: templates-clone
        image: alpine/git

You can't actually add another container to this in an override without copying the original. You'll have to do something like:

hub:
  initContainers:
      - name: templates-clone
        image: alpine/git
      - name: something-else
        image: alpine

This can get out of sync quite quickly.

In addition to treating extraContainers, etc as lists, we can also treat them as dicts, with name being the key. This lets helm overrides work as they should. We have done this in the past to extraConfig and friends in zero to jupyterhub, so doing this here would be valuble.

Alternative options

Do this in zero to jupyterhub, but that's probably the wrong place for this.

Who would use this feature?

Anyone doing a deployment of many hubs that are similar to each other.

[manics]

Do you think we should modify just these three properties, or should we do this for all List properties that aren't strings in one go?

[yuvipanda]

@manics ideally for all List properties, but we need to identify what the key should be for each of them. Makes it easy for containers, volumes, etc. The only other thing I can think of is env vars, but those are already settable as lists or dicts.