Add support for displaying milestones in the Now, Next, Later roadmap

What is the challenge?

Highlighting key upcoming milestones is important in PI Planning.

What is the impact?

The inability to display milestones makes the Now, Next. Later roadmap less usable than other roadmaps

Describe your idea

I would like to be able to display the milestones across the top of roadmap spanning the 3 columns. See the PI Roadmap on https://scaledagileframework.com/roadmap/ for a visual example.