Optionally notify of pending system updates

### Problem

Since upgrading to v8 I've frequently realized that I have a lot of pending system updates when I open System Settings->System->Operating System. In v7 you almost constantly had a little badge/bubble on the AppCenter icon in the dock, but now it feels like the pendulum has swung too far to the other side, i.e., you're never getting notified of pending system updates.

### Proposal

How about making it possible to opt-in for showing a badge/bubble on the System Settings icon in the dock whenever there are pending updates? The badge/bubble should also be shown for the System icon in switchboard, so that you could be led to System plug when opening switchboard.

### Prior Art (Optional)

_No response_