feat: add stream mode support

[AkisAya]

What type of PR is this?

fix:
add a workaround to preserve stream filed in user raw request, because openai-go's openai.ChatCompletionNewParams doesn't have this field, see issue #209

What this PR does / why we need it:

stream mode is not supported well, when user use an auto model, stream filed in of user raw input is dropped, so a non-sse response is returned , this pr reuse the ExpectStreamingResponse field introduced by this pr https://github.com/vllm-project/semantic-router/pull/203/files, to identify whether this request expect a stream response

1. detect if the request header Accept: text/event-stream
2. extract stream field from raw request body

and finally return a mutated request body to envoy with stream field

Which issue(s) this PR fixes:

Fixes #209

Release Notes: 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/extproc/request_handler.go

---

🎉 Thanks for your contributions!

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

[rootfs]

@tao12345666333 can you review this? thanks

[rootfs]

@AkisAya can you sign DCO?

Ensure you have a local copy of your branch by [checking out the pull request locally via command line](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally).
In your local branch, run: git rebase HEAD~2 --signoff
Force push your changes to overwrite the branch: git push --force-with-lease origin fix/stream-mode

[tao12345666333]

@tao12345666333 can you review this? thanks

Yes, please assign to me, I will review this PR tomorrow

[AkisAya]

@AkisAya can you sign DCO?

Ensure you have a local copy of your branch by [checking out the pull request locally via command line](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally).
In your local branch, run: git rebase HEAD~2 --signoff
Force push your changes to overwrite the branch: git push --force-with-lease origin fix/stream-mode

signed

[Xunzhuo]

have u tested when u set stream=true and got stream response?

[AkisAya]

have u tested when u set stream=true and got stream response?

sure.

you can get a stream response if your request body have stream: true or set a header Accept: text/event-stream

1. with stream field or with a header

2. without stream field or the header

[Xunzhuo]

tested it and working great, thanks!