Support 'geojson_text'::geometry

[Algunenano]

As suggested by Felix Kunde in the mail list, having geometry::json::text has created the need to be able to do the reverse, that is, parse automagically a geojson text into a geometry:

SELECT '{"type":"Point","coordinates":[1,1]}'::geometry

As long as { isn't a valid character in the other formats that we currently support as input (EWKT, HEXWKB, WKT), adding this piece of magic should be simple (make LWGEOM_in call lwgeom_from_geojson).

[Raúl Marín <git@…>]

In d92c6b7/git:

Support casting a geojson text to geometry

Closes ​https://github.com/postgis/postgis/pull/551
Closes #4656