cubicweb-vcsfile #4439400 Add a vcsinclude rest directive [validation pending]

That includes the content of a file from a Repository, like:

.. vcsinclude:: README
   :repository: <repo_eid>
   :revision: <revset>
prioritynormal
typetask
done in2.0.0
load left0.000
closed by#ade542c5723d [rest] add a vcsinclude directive (closes #4439400)
patch[rest] add a vcsinclude directive (closes #4439400) [applied]