Add sizes to circles in legend of circle and cluster layers

[otravers]

This was defined as a nice-to-have in https://github.com/mapbox/mapboxgl-powerbi/issues/210, not implemented as part of 1.4

As per this screenshot:

Or this Mapbox example (first screenshot): https://blog.mapbox.com/eviction-lab-map-617edfced73b

[otravers]

More relevant prior art:

The way Carto does it in this screenshot may also provide inspiration for https://github.com/mapbox/mapboxgl-powerbi/issues/313