support extra request params for byok model

### Describe the feature or problem you'd like to solve

some llm providers might require additional nonstandard request parameters for certain features or tweaks

### Proposed solution

env var `COPILOT_EXTRA_REQUEST_PARAMS='{"abc": "def"}'`
so `{"abc": "def"}` will be appended to the llm request body
similar features seems also exist in various other agent frameworks

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先定位組合 LLM 要求本文的 BYOK 程式碼，並檢查環境變數的讀取方式。加入將 COPILOT_EXTRA_REQUEST_PARAMS 解析為 JSON 並將其值附加至要求本文的支援，然後驗證有此變數和沒有此變數時的要求。

由索引模型根據 Issue 內容生成。