AlmostNever preemption policy - best effort and critical workloads in otherwise non-preemptible CQ

[mwielgus]

What would you like to be added:

New preemption policy - AlmostNever. Do not preempt unless the workload
belongs to one of 2 categories:

• BestEffort workloads - workloads that can be preempted by any workload.
• Critical workloads - workloads that can preempt any other workload.

Why is this needed:

Never rarely means never. Many Kueue users don't want preemption between regular workloads,
however, in case of low priority and super critical workloads, they want to make exceptions.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.