Support Hierarchical Cohorts with FairSharing

[mimowo]

What would you like to be added:

Proper support for Hierarchical Cohorts with Fair Sharing. Determine what are the gaps to make it officially supported and implement the support.

Why is this needed:

Currently the behavior for combining Hierarchical Cohorts with FairSharing is undefined. See:

kueue/apis/kueue/v1alpha1/cohort_types.go

Lines 68 to 70 in 3edad25

	// Cohort is the Schema for the cohorts API. Using Hierarchical
	// Cohorts (any Cohort which has a parent) with Fair Sharing
	// results in undefined behavior in 0.9

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• Implementation
• Docs update

The artifacts should be linked in subsequent comments.

[mimowo]

/assign @gabesaba
tentatively
cc @mwielgus @mwysokin @tenzen-y