[Map] Show marker for groups of smaller features at lower zoom levels

[gino-m]

Problem: Smaller polygons aren't visible when zoomed out, and appear only as several pixels on the map.

Also, multiple markers at the same location pile up and overlap.

Potential solution: At lower zoom levels we can group features using a marker with a number overlaid indicating the number of polygons at that location.

See also https://github.com/google/ground-platform/issues/673.