Support for fair sharing of unused resources

[mwielgus]

What would you like to be added:

Fair sharing of unused resources between ClusterQueues.

Why is this needed:

Currently Kueue distributes unused resources on "first come first served" basis. So users who submit their workloads at, say, 6 am may take over the whole unused capacity for the whole day (assuming that the other jobs have the same priority).

Completion requirements:

This enhancement requires the following artifacts:

• [ x ] Design doc
• [ x ] API change
• [ x ] Docs update

The artifacts should be linked in subsequent comments.

[mwielgus]

/assign @mwielgus