Add env variable to only enable social auth user registration

Description

Currently, the user registration process is enabled for both the built-in system and social auth providers. It would be nice to have the ability to allow only user registration through social auth providers.

Proposed Solution(s)

Introduction of new env variable that enables user registration only for social auth providers. Something like ENABLE_SOCIAL_AUTH_USER_REGISTRATION?

If the team would be open to this idea, I will create a merge request.