Support for widespread SQL types that falls into Jdbc type OTHER

There are several commonly used DB data types that don't have special treatment in the JDBC spec. Namely it's Geometry and JSON types that are supported by the most RDBMSs.

This task is about supporting those types in the Cayenne core (as well as in the dbimport). Initially we will support Wkt, GeoJson and JSON types.

See also discussion and some early prototype in this thread.