check for custom registration handlers

kamailio/registrar-role.cfg

@@ -175,6 +175,8 @@ route[HANDLE_REGISTER]
         return;
     }
 
+    route_if_exists("CUSTOM_HANDLER_REGISTER");
+
     #!ifdef PUSHER_ROLE
     route(PUSHER_ATTEMPT_REGISTRATION);
     #!endif
@@ -594,4 +596,6 @@ event_route[usrloc:contact-expired]
 #!include_file "registrar-sync-role.cfg"
 #!endif
 
+#!import_file "registrar-custom-handler.cfg"
+
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