Track Ray-native runtime architecture

[chokevin]

Summary

This tracks a Ray-native runtime direction for Prime-RL, separate from using RayJob only as a launch substrate.

Mainly using this as a POC and also because my stack is using Kuberay. Mainly will be maintaining it in a separate fork of mine.

[chokevin]

Opened a draft first upstream slice for the Ray-native direction: #2553. Scope is intentionally limited to the optional Ray rollout transport/config; the Ray-native runtime, Ray Train backend, and KubeRay examples stay as follow-up stacked PRs. Fork implementation was merged in chokevin#1.