Feature Request: Allow toggling plugins enabled/disabled

Currently, the `copilot plugin` command only allows for installing, listing, uninstalling, and updating plugins. There is no way to quickly toggle a plugin as enabled or disabled without completely uninstalling it.

Other AI CLI tools like Gemini CLI and Claude Code already support this functionality, which allows users to temporarily disable a plugin without losing its configuration or having to re-install it later.

**Proposed Change:**
Add `enable` and `disable` subcommands to `copilot plugin` (or a `--disabled` flag to `copilot plugin list` and a way to toggle).

Example:
```bash
copilot plugin disable
copilot plugin enable
```

This would be much more convenient than the current uninstall/reinstall workflow.