network-flow(donut): give the legend a component of its own

The legend is about to grow past what a v-for inside the chart can carry:
columns, a header of its own, and a second shape entirely for widgets with
no room for either. The chart's own job is the ring, so the split falls
where the two stop needing each other - the chart decides what the numbers
are, the legend decides how they read.

What crosses the boundary is a formatted row, not a slice: no total, no
percentage arithmetic and no formatValue on the legend side, which is what
lets a second legend variant consume the same rows without repeating any of
it. Hiding and highlighting stay with the chart, because the ring geometry,
the total and the center block are all read off them.

Nothing changes on screen. The class names take the new component's block
prefix, as the naming convention asks, and the tests follow them there.

CMK-37876

Change-Id: I251d59f0a10328e42bac71b91231a67e573ea19b