Consider making Culling a CRD

[andrewballantyne]

/kind feature

Why you need this feature:
[Is your feature request related to a problem? Please describe in details]
Culling Notebooks today is a configmap with some intentional open-ended structure.

We are moving to support more CRDs that make sense for the cluster. This may need a discussion at the Architectural Council or ADR to vet the API, but I believe we shouldn't communicate via a configmap of unstructured items.

Describe the solution you'd like:
[A clear and concise description of what you want to happen.]
New CRD (or possibly some other methodology that is structured, public API, or something else)

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]