cubicweb #10338621 allow usage of a metadata generator by the massive store [validation pending]

this abstraction is used by the nohook store and it's quite useful for projects that want to control metadata inserted at import time. Currently, those project can't work similarly with nohook and massive stores.

prioritynormal
typeenhancement
done in3.23.0
load left0.000
closed by#06eeac9389a3 [dataimport] introduce usage of MetadataGenerator into the massive store
patch[dataimport] introduce usage of MetadataGenerator into the massive store [applied][dataimport] introduce a new MetadataGenerator to replace former MetaGenerator [applied]