Implement RFC 50: OGR field subtypes. Impact core, OGR SQL, swig bindings, CSV, FileGDB, GeoJSON, GML, GPKG, OpenFileGDB, PG, PGDump, SQLite, VRT

From: Even Rouault <even.rouault@mines-paris.org>

git-svn-id: https://svn.osgeo.org/gdal/trunk@28008 f0d54148-0727-0410-94bb-9a71ac55c965