Add PostGIS variants of ST_Transform that take PROJ strings

[aseigo]

See https://postgis.net/docs/ST_Transform.html

There are three non-standard variants that take PROJ strings for projections. The first one works (magically) with GeoSQL.MM.transform/2, but the three-argument versions which take a to and a from projection do not.

These three-argument versions appear to be non-standard and PostGIS-specific, so should be implemented in GeoSQL.PostGIS