Allow remote auth user management

See https://jira.observium.org/browse/OBS-4335

We are migrating to remote auth (OIDC) due to several security reasons (and to enable SSO).

Authentication works all fine, but for the authorisation part we would like to have fine grained access, we want to assign user levels, roles, device or port permissions, ... for users provisioned after logged on, identically to how MySQL/RADIUS users are authenticated.

Not sure what is needed but first step would be to allow for a user insertion after authentication (cfr RADIUS), and then allow the permissions to be fetched from it (see function radius_auth_user_level)

We would use the SSO for customers as well so a user level of 1 is needed with a role assigned to each set of customers/departments.