cubicweb #2922322 make SQLGenStore aware of custom attribute types [open]

With CW 3.17, it's possible for a cube to define new attribute types that will be serialized in the SQL database with custom SQL types (array, hstore, ST_Geometry`, etc.). However, it's not possible to use the SQLGenStore of cubicweb to import such values because it's only able to serialize standard attribute types (e.g. Int, String, etc.) and doesn't provide any way to define how a new attribute type will be serialized for the COPY FROM statement.

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