Add async tool-enabled vLLM server for GRPO training via OpenAI-compatible interface

[BjarniHaukur]

What does this PR do?

This PR adds a new vllm_serve_async.py script to TRL. It:

• Enables asynchronous, OpenAI-compatible inference with vLLM
• Supports models that use tool calls (e.g., search APIs, python tool, general terminal usage)
• Mirrors the weight syncing logic from vllm_serve.py
• Delegates endpoint complexity to vllm.entrypoints.openai.api_server
• Exposes a rollout_func interface that lets users define custom input/output structures and tool definitions to forward into reward functions

Fixes #3284

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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[tmabraham]

thank you for this PR!!

[lewtun]

Hi @BjarniHaukur thank you for the PR! We're now looking to integrate environments in TRL, so would you like to rebase your branch on main so we can test your proposal more thoroughly?