fill-extrusion-flat-ceilings style spec property

Nobody has claimed this yet.

## Motivation

When using fill-extrusions on a map with terrain enabled, since #10347 the extrusion ceiling will be flat. This is good for buildings, but per https://github.com/mapbox/mapbox-gl-js/issues/10170#issuecomment-770496123 sometimes you might want these extrusions to follow the terrain.

One example is for vegetation extrusions to show forest areas of a fixed height above the terrain.

## Design Alternatives

The user could split the polygons but they still would be flat, just stepped. A smoother extrusion which follows the terrain could only be achieved my unlocking it with a style spec property.

## Design

Using a style spec property which defaults to flat allows backwards compatibility, but still supporting curved extrusion ceilings.

It's fine if this is not something you're keen on implementing, but thought I'd raise this ticket for further discussion since the existing ones were closed out.

Review the behavior introduced by #10347 and the discussion linked in the issue before deciding whether the proposed style-spec property is viable. Define how the property preserves flat ceilings by default while allowing terrain-following ceilings, then verify the style specification and fill-extrusion behavior against that design.

Written by the indexing model from the issue text.