This change adds the auto_link_user OmniAuth setting.
Users signing in to GitLab can be automatically linked to current user.
Users are linked based on email.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/24327
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36664
Related Omnibus MR: https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/4415