[serve][llm] Deliver raw HTTP requests to LLM engine

[eicherseiji]

Why are these changes needed?

• Serve deployment handle routers benefit from arbitrary headers on the raw HTTP request
• vLLM engine expects access to the raw HTTP headers for request ID

Propagation:

FastAPI Endpoint (router.py)
    ↓ request: Request
LLMRouter methods (chat, completions, embeddings, score)
    ↓ raw_request
_process_llm_request / _get_response
    ↓ raw_request
DeploymentHandle.remote(body, raw_request)
    ↓ raw_request
LLMServer methods
    ↓ raw_request
_run_request
    ↓ raw_request
LLMEngine methods (vLLMEngine)
    ↓ raw_request
vLLM OpenAI serving methods

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run pre-commit jobs to lint the changes in this PR. (pre-commit setup)
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[eicherseiji]

@kouroshHakha do you want to leave these here?