feat(option to strip default realm)

[Stratop0w]

add configuration option to add behavior similar to other external user authentication apps:

For Nextcloud instances using multiple domains / realms, configured with a 'default' domain, which is not required for a user to log in, this option will facilitate migration to user_backend_sql_raw and adoption.

This feature allow administrator to define a domain that will be stripped out from provided username, allowing users from that domain to autenthify without need to specify the domain part.