[FEATURE]: need to use consistent hashing for KV events in backend engines

[PeaBrane]

Feature request

The router may not update the radix tree properly in some cases, if the hashing algorothm used by the backend engines are not consistent across processes/nodes.

For example, we can set PYTHONHASHSEED = 0, or --prefix-caching-algo sha256 for vllm. Need to document this or showcase in examples.

Similarly, verify/document for sglang + trtllm + other engines.

Describe the problem you're encountering

N/A

Describe alternatives you've tried

No response

[zhaohaidao]

@PeaBrane Hi, I'd like to try it if no one else is working on it.

[WaelBKZ]

also interested in taking it if no one else does

[PeaBrane]

@zhaohaidao please go for it!

@WaelBKZ feel free to also coordinate and work together!

[qimcis]

Since I didn't see any updates here, I've submitted a PR! Let me know if I misunderstood anything

[PeaBrane]

@qimcis shape looks good, thanks! Left some comments.