Display 3D GeoJSON data in HsCesium

[jmacura]

The GeoJSON format is not very suitable for storing data in 3D.
The standard somewhat allows to store 2.5D data in a form [lon, lat, elevation] or [lon, lat, altitude]. This is ambiguous as altitude ("above ground") and elevation ("above mean sea level") are different concepts. The GeoJSON Format. We can place the point/polygon/line to that height, but we must pick one of the two options, there is not guesstimate to differentiate elevation from altitude. From a quick googling, it seems to me that the "elevation" is somewhat more common as the "z" coordinate, e.g. emphasized by tmcw.
Additionally, the information about e.g. object's height can be stored in any property (e.g. "height"), which can be used to extrude the polygon/line into that height.

It is possible to do some fancy stuff with geometries in Cesium, like in this example: https://sandcastle.cesium.com/index.html?src=Polygon.html but it is not straightforward how to do it with a data in GeoJSON format...
It is about how to access/create PolygonGraphics from the GeoJsonDataSource before/while the entities are created.

[jmacura]

In cesium-fixes-14 branch, it was proven as feasible:

yet some bugs remain, which need to be treated.

[jmacura]

Resolved in #4749

Cesium's own approach with "extrudedHeight" was followed, so if the layer contains "extrudedHeight" property, it is used to extrude the polygon/line into the specified height (above MSL).