HierarchicalCohort: Support validations to avoid accidentaly Parent Cohort removal

[tenzen-y]

What would you like to be added:
We would like to add a validation webhook for the Cohort deletion operation.
The validations verify if the Cohort is not referred to by other Cohorts as a parent.

Why is this needed:
Currently, cluster admins can remove the Cohort even if the Cohort is referred to in other Cohorts .spec.parent field.
However, the deletion operation is not appropriate.
Ideally, the cluster admin should change the parent in the child Cohorts first before they remove the parent Cohort.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[tenzen-y]

/cc @mimowo @gabesaba