Model setting specific to commit message generation

We currently have multiple settings for chat models:
- `chat.defaultModel`
- `chat.utilityModel`
- `chat.utilitySmallModel`

Commit message generation uses `chat.utilitySmallModel`. **However**, the utility small model isn't only tied to commit message generations, so changing it influences other areas of the product.

Given issues such as https://github.com/microsoft/vscode/issues/316204 might arise from time to time, and given users may want finer control over which model generates messages (e.g., I like GPT 5.4-mini style, even if it takes more time and costs more credits), the idea is to have a setting specific to this functionality.

For example: `github.copilot.chat.commitMessageGeneration.model`

Old related issue: https://github.com/microsoft/vscode/issues/307634