[POC]: feat: add Kueue prebuilt workload backend

[alaypatel07]

Add a Kueue scheduler backend path that creates prebuilt Workloads from Grove PodGangs so PodGroup minReplicas can map to Kueue PodSet minCount. Include focused demo manifests for standalone minCount and PCSG all-or-nothing behavior.

What type of PR is this?

What this PR does / why we need it:

This PR adds Kueue integration. This PR:

1. adds kueue scheduler backend
2. the backend pre-creates the workload for the pods
3. the operator then creates the pod with specific labels that is needed
   for kueue to admit the workload.

Which issue(s) this PR fixes:

Fixes #704

Special notes for your reviewer:

Does this PR introduce a API change?

GREP-704: add kueue integration

Additional documentation e.g., enhancement proposals, usage docs, etc.:

TBD

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[shayasoolin]

@enoodle would you like to review this? Thanks