Progressbars for Dock Bands

### Description of the new feature / enhancement

Command Palette Dock Extensions should be able to set a progress percentage to indicate the progress of a background process or event.

### Scenario when this would be used?

It could be used to indicate the progress of a download, a countdown or real world events such as a football match, food delivery or flight arrivals.

### Supporting information

I imagine it to be similar to the progress bars on the windows taskbar except maybe using custom colours or more subtle colours, like dark grey for a dock that is coloured black etc.

Start by locating the Command Palette Dock Extensions entry point and reviewing how dock bands are currently exposed to extensions. Define the supported progress-percentage behavior and determine whether custom colors are in scope; the work is done when an extension can visibly report progress for the described background-process scenarios.

Written by the indexing model from the issue text.