Applying styling to to BarChartGroupData

**Is your feature request relasted to a problem? Please describe.**
I'm showing data based on months (i.e x axis is months, y axis is value) - I've got three rods per month, and I have various logic to determine the color. I'm looking at options on how to highlight a specific month (e.g the current month). This is only for aesthetic purposes.

**Describe the solution you'd like**
Since my implementation is using colors, my only option appears to be the rod width to highlight rods. It would be great if there were additional options available and a couple of options I could think of :
- Applying a border to BarChartGroupData (e.g dotted, dashed, solid)
- Applying an elevation to the rods to pop it out

**Describe alternatives you've considered**
Alternatives I've tried is to alter the color codes, adjusting the rod sizes (rod width)

Start by reviewing the BarChartGroupData API and the existing bar styling options in fl_chart. The issue names borders and elevation as possible solutions but does not identify files, tests, or a settled design; clarify the desired API and define completion around a supported way to highlight a month or group.

Written by the indexing model from the issue text.