feat(transit-vehicle-overlay): Add optional per-vehicle color

[mcleinman]

Allow optional per-vehicle colors in transit-vehicle-overlay. This is not a breaking change; it will continue to default to the color provided to the entire object if a vehicle does not have a valid color. I also updated one of the stories to show this new functionality.

This is similar to #442. We're likely going to update to a non-raster-tiles version, so we need this functionality in the main branch as well.

[binh-dam-ibigroup]

Allow optional per-vehicle colors in transit-vehicle-overlay.

Hi @mcleinman, thanks for this PR. By curiosity, could you provide a clarification of what the highlight color on one vehicle, but not others on the same transit route, is intended to mean? Thank you!