[Feature] Support piggyback server load report.

Checklist

• 1. If the issue you raised is not a feature but a question, please raise a discussion at https://github.com/sgl-project/sglang/discussions/new/choose Otherwise, it will be closed.
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Motivation

To support a flexible and reliable load balancing strategy, external router (sgl-router) and internal controller (dp-controller) should be aware of the real-time load situation.

Current implementation uses a polling mechanism and a watch-based communicator to query load from the scheduler. However, this is flexible and elegant.

• Implement a piggyback load reporting mechanism, in which load information is carried through restreaming token messages.
• Also keep the watching mode, always storing the latest load information in http worker for external query.

Related resources

No response