public void add_feature_by_type (Type feature_type)
You can use this when you don't need to customize the new feature in any way.
feature_type is not a SessionFeature type, this gives
each existing feature on this the chance to accept
feature_type as a "subfeature". This can be
used to add new Auth or
Request types, for instance.
You can also add a feature to the session at construct time by using the
See the main Session documentation for information on what features are present in sessions by default.