If feature_type is the type of a class that implements [iface@SessionFeature], this creates a new feature of that type and
adds it to this as with [method@Session.
add_feature]. You can use this when you don't need to customize the new feature in any way. Adding multiple features of the same
feature_type is not allowed.
If feature_type is not a [iface@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 [class@Auth] types, for instance.
See the main Session documentation for information on what features are present in sessions by default.
| this |
a Session |
| feature_type |
a Type |