Custom resource links

[thejoeejoee]

Is your feature request related to a problem? Please describe.
Currently, the <ENTER> press is handled for some resources by displaying related resources.
For all the other resources, the <ENTER> press is handled by displaying the resource describe.
That's not intuitive for custom resources, where some kind of relations might be defined - e.g. ExternalSecrets from ESO and related secrets.

Describe the solution you'd like
I'd like to have an option to define a custom behaviour for the <ENTER> press based on the selected resource.
It would be based on source and target GVR, also with some kind of relation/filtering definition.

Describe alternatives you've considered
As relations like Deployment -> Pods are already built-in in k9s, not all resources could be handled in the same way.
There's custom resource definitions, and it's not possible to define all of them in k9s, especially those close-sources.

Additional context
The definition could look like this, as is currently implemented in #2562:

external-secrets.io/v1beta1/externalsecrets:
  target: v1/secrets
  fieldSelector:
    metadata.name: .spec.target.name

cluster.k8s.io/v1alpha1/machines:
  target: v1/pods
  fieldSelector:
    spec.nodeName: .metadata.name

custom.io/v1/customparents:
  target: v1/customchildren
  labelSelector:
    parent: .metadata.name
    type: .spec.type