Metadata-level support of different coordinate systems

[hombit]

Catalogs are distributed in a variety of different coordinate systems, which are not necessary supposed to be converted into ICRS. It would be nice to generalize coordinate system support of hipscat: allow arbitrary longitude / latitude pairs, support storage of coordinate system metadata: name, type, epoch, coordinate names, etc.

This issue is not talking about coordinate system transformations, see astronomy-commons/hats-import#142 for that