feat: add HNSW index to inmemory semantic cache

[rootfs]

What type of PR is this?

What this PR does / why we need it:

The inmemory cache search is not optimized, adding HNSW index will accelerate the search

This is the benchmark result:

cc @aeft

Which issue(s) this PR fixes:

Fixes #

Release Notes: Yes/No

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 src

Owners: @rootfs, @Xunzhuo, @wangchen615
Files changed:

• src/semantic-router/pkg/cache/comprehensive_benchmark_test.go
• src/semantic-router/pkg/cache/cache_factory.go
• src/semantic-router/pkg/cache/cache_interface.go
• src/semantic-router/pkg/cache/inmemory_cache.go
• src/semantic-router/pkg/cache/inmemory_cache_integration_test.go

📁 config

Owners: @rootfs
Files changed:

• config/config.development.yaml
• config/config.yaml

📁 website

Owners: @Xunzhuo, @rootfs, @yuluo-yx
Files changed:

• website/docs/tutorials/semantic-cache/in-memory-cache.md

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🎉 Thanks for your contributions!

This comment was automatically generated based on the OWNER files in the repository.

[rootfs]

closing and replacing it with #504