TAS: API to support rank-based ordering for custom CRDs

[mimowo]

What would you like to be added:

API which allows to use custom PodIndex labels for custom CRD jobs, without the incentive to use labels reserved for kubernetes in the in-house Jobs.

Why is this needed:

• the current PodIndex labels lookup only work for "known" jobs with built-in integrations: here
• this will not work for in-house Jobs, unless the developers use the labels from other projects (like kubernetes.io/job-completion-index), which creates an unhealthy incentive in the long run
• This is part of the graduation plan for stable: https://github.com/kubernetes-sigs/kueue/tree/main/keps/2724-topology-aware-scheduling#stable.

Completion requirements:

• [ ] KEP update
• [ ] API change
• [ ] Docs update

[mimowo]

The proposal is to extend the workload PodSetTopologyRequest API with the following fields:

// PodIndexLabel indicates the name of the label indexing the pods. 
// For example, in the context of
// - kubernetes job this is: kubernetes.io/job-completion-index
// - JobSet: kubernetes.io/job-completion-index (inherited from Job)
// - Kubeflow: training.kubeflow.org/replica-index
PodIndexLabel *string

// SubGroupIndexLabel indicates the name of the label indexing the instances of replicated Jobs (groups)
// within a PodSet. For example, in the context of JobSet this is jobset.sigs.k8s.io/job-index.
SubGroupIndexLabel *string

// SubGroupIndexLabel indicates the count of replicated Jobs (groups) within a PodSet.
// For example, in the context of JobSet this value is read from jobset.sigs.k8s.io/replicatedjob-replicas.
SubGroupCount *int32

The values could be then set when implementing the PodSets() function in the GenericJob interface via the PodSetTopologyRequest helper function like here.

Then, the API could be read from TopologyUngater, instead of the lookups.

[mimowo]

cc @PBundyra @tenzen-y @mwielgus @mwysokin