preferred_username as default field representation in OIDCUser model

[fabiothz]

The preferred_username field is always used to represent the user, and a username or email can be chosen to fill in this field.

If the admin chooses not to use the username field as an email, allowing users to register with just a username, without filling in the email field, the representation of the OIDCUser model does not show this user identification.

Therefore, I propose that the preferred_username field be the field to be shown in the representation of the OIDCUser model

[code-specialist]

Definitely an improvement regarding clarity and generality. Thank you for this proposal Fabio.