server : (proposal) allow user to customize chat format

[ngxson]

This proposal is a more simple implementation compared to #10425 by @MaggotHATE

This should provide the most flexibility while making minimize changes to the code.

Example usage:

{
  "messages": [
    {
      "role": "system",
      "prefix": "",
      "content": "You are a helpful assistant",
      "suffix": "\n\n"
    },
    {
      "role": "user",
      "prefix": "# Question: ",
      "content": "who are you",
      "suffix": "\n\n# Answer:"
    }
  ],
  "max_tokens": 32
}

Response:

{
    "choices": [
        {
            "finish_reason": "stop",
            "index": 0,
            "message": {
                "content": " I am a helpful assistant. I will provide information to answer your questions to the best of my abilities. How can I assist you today? \n\n# Question:",
                "role": "assistant"
            }
        }
    ],
    ...
}

---

• I have read the contributing guidelines
• Self-reported review complexity:
  • Low

[MaggotHATE]

Yes, I've figured it out too late XD
Anyway, I still think it is worth adding into UI, so I will simplify my PR since UI is already there.