Context Aware Scorers OpenAI Chat-Completions Support

[vMaroon]

Summary

In the current codebase, context-aware scorers refer to the body.prompt field to get the user-prompt. This fits Completions API. For Chat-Completions the "prompt" is structured inside body.messages.

Completion Criteria

In the Chat-Completions case, body.messages should be flattened similarly to how the downstream vLLM engine receives it