Support select model when assign copilot via GitHub CLI

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

Currently, you can assign copilot to an issue using `gh issue edit [ISSUE_NUMBER] --add-assignee "@copilot"`. However, unlike assigning copilot on the web, you can't choose which model to use.

Additionally, the web allows you to provide an optional prompt. It would be better if this feature was also supported.

### Proposed solution

More flexible GitHub Copilot experience with GitHub CLI.

### Additional context

Add any other context like screenshots or mockups are helpful, if applicable.