[req] add a _set_user method RequestSessionBase usally need to recreate a new CWuser appobject linked to themself. We add it to the base class to avoid multiple redifinition.

[server/session] add a login property session.login is a DBAPISession attribute. Having it on server side session will helps the rework of the API to access repository. The new schema drop the concept of DBAPISession and use server side session for the same purpose. related to #2503918

[server/session] allow access to session id using sessionid session.sessionid is a DBAPISession attribute. Having it on server side session will helps the rework of the API to access repository. The new schema drop the concept of DBAPISession and use server side session for the same purpose. related to #2503918

[server/session] Implement anonymous_session Now we have a simple rule to compute if a session is anonymous we can implement the property for server session too. Having it on server side session will helps the rework of the API to access repository. The new schema drop the concept of DBAPISession and use server side session for the same purpose. Related to #2953943 Related to #2503918

[server/session] do not clear session.cnx if unrelated to cnx actually cleared The cnx parameter may be different from the one actually loaded for the current thread. This will be possible in future commit when you close a client connection for example.

[dbapi] move ProgrammingError into cubicweb module A new ``repoapi`` will be introduced as a replacement for the dbapi. It will need ProgrammingError too. Related to #2503918