Make generation of xml:id attribute configurable through an adapter
The idea is to have the generation of xml:id attribute configurable based on entity context. So we create an entity adapter IXmlId that default to the current implementation. This will be extended in cubicweb-saem_ref where we'd want to use the ARK identifier to produce this xml:id attribute. In profile_generation module, we rename eid2xmlid function as xmlid and make it accept an entity.