[service-api] Add unified service API on `_cw` attribute This will allow writting generic entity code that could run on both client and repo side. This addition have been discussed with Sylvain Sylvain Thenault.

# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
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%(long-license)s
"""%(distname)s entity's classes"""