cubicweb-dataio #3216724 XY import and entity callback [validation pending]

Some RDF attributes may be transformed in a specific Etype in the Yams schema (e.g. dc:title -> Name), and we should be able to pass callbacks in XY for this.

prioritynormal
typebug
done in0.3.0
load0.500
load left0.000
closed by<not specified>