cubicweb-postgis #5313835 Add support for litteral WKT strings [validation pending]

Basically we need to add bindings for ST_GeomFromText() and ST_GeogFromText(). It would then allow writing RQL queries such as

Any Z, ZN
WHERE Z is Zone, Z name ZN, Z type "Commune", Z geometry G
HAVING ST_CONTAINS(G, ST_GEOMFROMTEXT("POINT(651789.5 6862043.0)", 2154)) = TRUE

In the other direction, we also need to return a Geometry as WKT, that is we need to add support for ST_AsText() function, thus allowing queries like the following:

Any N, ST_ASTEXT(G) WHERE F is Feature, F eid 123, F name N, F geometry G
prioritynormal
typeenhancement
done in0.4.0
load0.500
load left0.000
closed by#7509aa8c54b2 Register function to get geometry as WKT., #eb8c40a52717 Add various GIS functions to RQL functions (closes #3426092)
patchRegister function to get geometry as WKT. [applied]Add various GIS functions to RQL functions (closes #3426092) [applied]
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