[adapters] Use an adapter for serializing entities to a bare python structure which may be used for e.g. JSON export or other and allow to have different serializations for each entity types. Closes #6378035

usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py