cubicweb-datacat #5511208 Harvest uData REST API [in-progress]

Should work on

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patchHarvest uData REST API [reviewed]Compute DCAT URL for Distribution ext entities coming from uData [reviewed]Fetch ext entities from uData REST API [reviewed]Add uData JSON decode function [reviewed]Rename DCAT RDF feed parser. [applied]Move ext entity cleaning out of fetch function. [applied]Make ExtEntity valid literal function use attr def instead of target schema. [applied]Function converting to Geometry now handles GeoJSON. [rejected]Fix datetime base converter. [applied]