Add Security Manager environment variable and access value

Summary

Add the foundational infrastructure for the Security Manager role (access level 25) without exposing it publicly. This is Phase 1 of implementing the new Security Manager role.

Implementation Details

• Add SECURITY_MANAGER = 25 constant to lib/gitlab/access.rb
• Add environment variable check ENV['GITLAB_SECURITY_MANAGER_ROLE'].
• Create lib/gitlab/security_manager_config.rb with centralized feature detection
• Keep existing public methods unchanged
• Add private helper methods for future phases

Acceptance Criteria

• Gitlab::Access::SECURITY_MANAGER constant is defined
• Role is not exposed in public APIs unless the env var is enabled
• No breaking changes to existing functionality
• Environment variable defaults to disabled

Environment Variable

GITLAB_SECURITY_MANAGER_ROLE=true to enable (defaults to false)