cubicweb #2203418 implement a generic way to call services server-side from the web-side [validation pending]

this is at least needed by cubes like vcsfile to refresh a vcsrepo from the web ui

prioritynormal
typeenhancement
done in3.15.0
load0.300
load left0.000
closed by#c9babe49c1c1 [repository] implement a generic way to call repo-side services; closes #2203418