Resolve "Add Security Manager role infrastructure"

What does this MR do and why?

This code change adds a new "Security Manager" user role in GitLab, which can be enabled or disabled via an environment variable. When enabled, this role sits between the "Reporter" and "Developer" roles in the access level hierarchy and provides comprehensive visibility and management over security aspects of projects and groups. The implementation includes proper testing to ensure the role appears in all the right places when enabled and is completely hidden when disabled, maintaining the existing role order and functionality.

References

Related to #582639

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