Add System Prompt parameter for Copilo-CLI

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

There is no way to give system level instructions to Copilot-CLI outside of repo specific instruction files.

### Proposed solution

Add a `--system-prompt` parameter for copilot-cli to pass system level instruction directly on top of the repo level instruction files.

### Example prompts or workflows

`copilot -p "make so cool stuff" --system-prompt "extra instruction on making cool stuff"`

### Additional context

_No response_