[Feat] Prefix-aware routing and load balancing

[KuntaiDu]

This PR implements two prefix-aware routing algorithms and a simple load balancing logic in #59 .

This PR introduces two set of abstractions: affinity and endpoint_filter. PTAL and see if you are good with these APIs.

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[BrianPark314]

great work, one nitpick comment is that the affinity and endpoint_filter might be better placed under services/routing_service.

[gaocegege]

It is a great start, maybe we could implement more intelligent routing logic like https://docs.sglang.ai/router/router.html#cache-aware-load-balancing-router

[KuntaiDu]

It is a great start, maybe we could implement more intelligent routing logic like https://docs.sglang.ai/router/router.html#cache-aware-load-balancing-router

Yeah we should have it in the future! For now let's have some simple routers that make no assumption on the LLM inference engine.