Implementation of simple load balance routing proxy server (vllm-project#1953) (vllm-project#2124)

### What this PR does / why we need it?
The PR is the cherry-pick from v0.9.1
vllm-project#1953

This PR introduce a new load balance proxy server example implementation
for disaggregated pd, which support simple token&kv_cache aware load
balance routing strategy for the disaggregated pd system compared with
origin round robin toy_proxy.

### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
tested on real workload and unittest

- vLLM version: v0.10.0
- vLLM main:
vllm-project/vllm@ad57f23

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Signed-off-by: ganyi <[email protected]>

Author: ganyi1996ppo