feat(slime): Add integration of slime + rollout gateway.

[Linbo-Liu]

Issue #, if available:

Description of changes:

Summary

• Adds backends/experimental/slime/integration/ with two slime hook entry points:
  • rollout.generate (--custom-generate-function-path): per-sample ACR rollout via RolloutGateway + RolloutClient; maps trajectory leaves to slime Samples; failures return ABORTED with remove_sample=True
  • rewards.normalize_episode_rewards (--custom-reward-post-process-path): GRPO normalization that groups by group_index + dedups by rollout_id, fixing slime's positional reshape when a rollout forks into multiple leaves
• Adds GSM8K example (examples/math_agent/) with train.sh, config.yaml.example, SETUP.md, and install_slime.sh

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