planning: support more fields on /chat/completions (for OpenAI Compatiblity)

[gabrielle-ong]

Problem
Roadmap ticket for /chat/completions fields for full OpenAI Compatibility / Enterprise roadmap
They cannot be supported directly with cortex.cpp
followup ticket from #1582 (comment)

• store boolean:
  • for https://platform.openai.com/docs/guides/distillation
  • https://platform.openai.com/docs/guides/evals
• metadata object
  • https://platform.openai.com/chat-completions
• service_tier string
• modalities (+ audio) :
  • https://platform.openai.com/docs/api-reference/chat/create#chat-create-modalities
• user: https://platform.openai.com/docs/api-reference/chat/create#chat-create-user.

Success Criteria

Tasklist

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