Provide manual model selection for AI-generated git change suggestions

I noticed that the AI suggestion generation logic in Git currently utilizes the model used in the previous round of conversation on Copilot, which undoubtedly increases the cost of use (even if the BYOK path is used). Therefore, I believe that providing a manually used model in Git's AI suggestion generation would be a reasonable idea

我注意到，目前git的ai建议生成逻辑是使用的copilot上一轮对话中使用的模型，这无疑是会增加使用成本的（即使使用BYOK路径），所以我认为，在git的ai建议生成中，提供一个手动使用的模型将会是一个合理的想法