[VSA][FE] Add additional information to stage popovers

We've implemented a MVC for adding explanation about what each stage in VSA represents in #300235 (closed) including the following information:

• Start event (backend issue, frontend issue)
• Stop event (backend issue, frontend issue)

Proposal

In group-level VSA, the popovers for horizontal stages should show additional information:

• Average (backend issue: #273208 (closed))
• Median (backend already exists)

Items in stage will be added to the popover in #321448 (closed) (backend issue: #324687 (closed))

Mockup