Add runtimeClassName support for GPU workloads

[oren-openteams]

Summary

• Add optional runtimeClassName field for head and worker pod specs
• Enables GPU support via runtime classes (e.g., nvidia) on k3s and other runtimes
• Document GPU configuration in README

Usage

image:
  repository: rayproject/ray-ml
  tag: 2.43.0-py39-gpu

worker:
  runtimeClassName: nvidia
  resources:
    limits:
      nvidia.com/gpu: "1"

Test plan

• Verify chart renders correctly without runtimeClassName set (no change to existing behavior)
• Verify chart renders correctly with runtimeClassName set
• Test GPU inference on a cluster with NVIDIA GPUs

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