Is your feature request related to a problem? Please describe.
There are often default options or configurations that we need to set for users Notebook pods such as environment variables, or configmaps/secrets that we want mounted to every users pod. There currently isn't a way to set any sort of global or default options for many of these settings.

Describe the solution you'd like
The upstream Kubeflow project supports an object called a PodDefault which can be used to configure different default options when starting a Notebook with Notebook Controller. This is a great replacement for what we used to be able to do with the SingleUserProfiles in JupyterHub.

Describe alternatives you've considered

Additional context
The kubeflow documentation shows an example of using PodDefaults in step 11 here:

https://www.kubeflow.org/docs/components/notebooks/quickstart-guide/