[Proposal] Add progress bar radius property

[navaronbracke]

When the stroke cap is StrokeCap.round, the progress bar is painted with a fixed radius that is half the bar height.

Allow overriding the default by adding a new property double? progressBarRadius,
which defaults to barHeight / 2 if null.

[suragch]

Currently the stroke cap maps directly to the Paint stroke cap for drawing a line. There isn't a radius option with that that I'm aware of. This could be changed by painting a rectangle with rounded corners. Currently I don't have a plan to do that, so I would recommend you just copy the source code to your project and modify the _drawBar method to meet your needs.