ClusterQueue should report lended resources

[avrittrohwer]

What would you like to be added:

Report lended resources in ClusterQueue status

Why is this needed:

For Kueue setups were one ClusterQueue holds all quota with many ClusterQueues borrowing, it is difficult to determine how many resources are currently utilized. It is technically possible by summing up all the borrowed resources from the borrowing cohorts but this is tedious. It would be much simpler if the ClusterQueue lending quota reported how much it was lending.

Completion requirements:

Make it easy to determine how much nominalQuota is being used by borrowing ClusterQueues by running kubectl get / describe on the ClusterQueue that owns the quota.

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.