(feat) Support chunked prefill with prefix cache

[GindaChen]

This PR is a checkpoint of some features we want in our branch.

Support prefix caching with chunked prefill.

Testing.

• Open 2 vllm server with --enable-prefix-caching and --enable-chunked-prefill.
• Compare the output using the same prompt in api_client.py and see if they match.

## Server side
# Round 1: without prefix caching
sh scripts/vllm-server.sh 

# Round 2: with prefix caching
sh scripts/vllm-server.sh --enable-prefix-caching

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which consists a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of default ones by unblocking the steps in your fast-check build on Buildkite UI.

Once the PR is approved and ready to go, please make sure to run full CI as it is required to merge (or just use auto-merge).

To run full CI, you can do one of these:

• Comment /ready on the PR
• Add ready label to the PR
• Enable auto-merge.

🚀

[GindaChen]

See discussion (of correctness) in

• https://github.com/vllm-project/vllm/pull/6819/files
• https://github.com/vllm-project/vllm/pull/6144/files#diff-cb3ec64fe03957558abc2120f7d83d3eea9d4921600ab5382c3938e314a890da