Topic/default/rql schema holder

Add two views that can serve the schema or it hash to a cwclientlib client.

The schema and the hash are stored in a cached property for optimisation.

cubicweb_rqlcontroller/__pkginfo__.py

@@ -16,7 +16,6 @@
 __depends__ = {
     'cubicweb': '>= 3.27.3',
     'six': None,
-    'cached_property': None,
 }
 __recommends__ = {'cubicweb-signedrequest': None}