[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.
fix ``call_service`` API and add test service API
``call_service`` was taking a session argument. But Service is an AppoObject and
already has the same session accessible as ``self._cw``.