Add taxonomy field type to user attributes

Problem: Many sites use taxonomies instead of list fields. For example "Work Location" could be a taxonomy of a company's different office locations.

Currently these can not be passed as a CAS attribute as a taxonomy item, but could be passed as a text list field.

Mapping of the taxonomy fields could have a UI similar to the user roles feature.

It would be very useful to be able to synchronise user accounts between CAS sites. My usecase is taxonomies, but perhaps the feature request should be broader - "add support for all user field types"?

Apologies - no patch attached. It is currently well outside of my competencies to create one!