Grey hard-coded in BreakdownTags

[ryandle]

Information

• OS: Windows
• Version: 0.43.0
• Terminal: cmd.exe

Describe the bug

I noticed that there is a hard-coded grey color inside the breakdown chart tag format: https://github.com/spectreconsole/spectre.console/blob/main/src/Spectre.Console/Widgets/Charts/BreakdownTags.cs#L58

It can be difficult to see on light backgrounds:

To Reproduce
Display a breakdown chart on a light or grey background.

Expected behavior
I'd expect this value to either be not colored by default or be configurable.

However, I'm guessing now there would be a back-compat concern around changing it, so perhaps the fix is to keep grey the default but expose a way to configure it with a new field on IBreakdownChartItem.

Screenshots
See above

Additional context
I'm working on incorporating Spectre.Console into Maoni's realmon tool at Maoni0/realmon#39 and ran into this issue while coming up with a light color theme.

Happy to contribute a fix here if there is an acceptable resolution! Thanks for this wonderful library.