[cwuser] make CWUser callable, returning self for dbapi compatibility In the dbapi, Connection and Session have a ``user`` method to generated a user for a request In the repo api, Connection and Session have a user attribute inherited from SessionRequestBase prototype. This ugly hack allows to not break user of the user method. To ease transition from dbapi to repoapi we need the ClientConnection to be as compatible as possible with the dbapi. Adding this method goes in this direction. It'll get deprecated in the deprecation wave that will conclude the repoapi refactoring. related to #2503918

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