[new feature] billboard / label background outline color

Hey

I would like to add an outline color to labels background.

since labels are using billboard, we actually need to support `backgroundOutlineColor` to `Billboard`

I also be happy to add a PR with some directions in how to implement that.

Thanks

Start by locating the Billboard implementation and the label code that uses billboards, then trace how the existing background color is represented and rendered. Confirm the intended API name, backgroundOutlineColor, and identify the relevant rendering test or example if present. Done means labels can display the requested background outline color without changing existing label appearance by default.

Written by the indexing model from the issue text.