[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.
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<script src="../../data/cubicweb.js" type="text/javascript"></script>
<script src="../../data/cubicweb.python.js" type="text/javascript"></script>
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<h1 id="qunit-header">cw.utils functions tests</h1>
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