Resolve "Undefined omniauth_authorize_path when omniauth_enabled not enabled"

What does this MR do?

Also setup omniauth when omniauth_auto_sign_in_with_provider is set.

Are there points in the code the reviewer needs to double check?

It feels weird that we have both Gitlab::Auth.omniauth_setup_providers and Gitlab::OmniauthInitializer.new(config).execute setting up omniauth, we should probably somehow merge them.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48677 (closed)