Add support for dict-based profile list definitions to zero-to-jupyterhub-k8s

[agoose77]

This PR jupyterhub/zero-to-jupyterhub-k8s#3690 adds support for dict-based configuration of list items in zero-to-jupyterhub-k8s. It is not exhaustive, and in particular does not add this option to profile list definitions.

In a recent conversation about how to design hubs, @yuvipanda confirmed that we want to lift as much configuration into common configuration files (per cluster) as possible. By adding support for defining profile lists as dictionaries, it will be possible to define extensible profile lists in the common configuration.

Definition of Done

• A PR has been authored that adds support for dict-based profile lists
• The PR has been tested
• The PR has been reviewed
• An issue has been created to track applying this change to our clusters

[yuvipanda]

Indeed, this requires changes to kubespawner that I thought we already had done but we hadn't.

[yuvipanda]

@agoose77 can you scope out what changes would actually be needed to be done upstream? Just kubespawner? Kubespawner and z2jh? Spend no more than 15min on sizing it too.