mixer: new extension that uses Mixer library

• If the extension is enabled it will connect to database create and recreated commands so it can loads all the json files into the database.

• NOTE if this extension is enabled fixtures should not be used as probably there will be some database inconsistencies.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>