Use ClusterQueueReference to improve static code analysis

[mimowo]

What would you like to be added:

I would like to review all places in code which use string to represent ClusterQueue and replace with ClusterQueueReference. Some candidate places:

https://github.com/kubernetes-sigs/kueue/blob/6e25ebfcf2491236e385f644e4f7280c3d315ead/pkg/cache/snapshot.go#L39C2-L39C26

kueue/pkg/hierarchy/manager.go

Line 23 in 6e25ebf

ClusterQueues map[string]CQ

but likely many more. It is ok to miss some, but we would like to set the direction.

Why is this needed:

To make it easier to analyze the code and see what the "string" is.

[mimowo]

Follow up to #4417

cc @vladikkuzn @PBundyra @tenzen-y