Add support for compound CRSs

[cportele]

See opengeospatial/ogc-feat-geo-json#102

With 3D data, there is often not a single, registered CRS available and a compound CRS has to be used combining a 2D CRS with a vertical CRS.

This is currently not supported by Part 2.

JSON-FG also supports such compound CRS and software that supports CRSs often supports such compound CRSs, too.

Adding support for compound CRSs should be done maintaining backwards compatibility, i.e., the existing properties "storageCrs" and "storageCrsCoordinateEpoch" should stay as they are.

A new property with an extensible content model should be added for more complex cases. Future extensions could also support the CRS definition inline, not just by reference. The JSON-FG schema could be used for the value, for example.