core: added a mechanism to associate short names to namespaces:

- new internal registerNamespace can be used by a plugin to associate a short name to a namespace
- new NamespacesGet bridge method retrieve those associations

src/bridge/bridge_constructor/bridge_template.ini

@@ -763,3 +763,11 @@ doc_param_0=%(doc_profile_key)s
 doc_return=session informations, with at least the following keys:
     jid: current full jid
     started: date of creation of the session (Epoch time)
+
+[namespacesGet]
+type=method
+category=core
+sig_in=
+sig_out=a{ss}
+doc=Get a dict to short name => whole namespaces
+doc_return=namespaces mapping