User story #22206: Allow user to define custom roles in rudder-user.xml
Unknown roles must be ignored, not lead to invalid role
Currently, if a custom-role is defined and it reference unknown permission, then it is ignored.
We want to allow that (and just ignore the unknown permissions) so that, for ex if a role refers perm related to other plugins, the role remains OK even if one of the plugin is not loaded (and so its permission are unknown from Rudder).
This does not create a security risk since our permission are only additive (union based), and there is no "exclude".