Add preferred_username support (OIDC provider) (#420)

* Add support for preferred username.

* Add missing TOC entries.

* Add note about preferred_username support.

* Adjust tests.

* Check on not implemented error for GetPreferredUsername() call.

Co-authored-by: Felix Fontein <felix@fontein.de>
Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>