Add Security Manager GraphQL schema support

Summary

Add GraphQL schema support for the Security Manager role, conditionally exposed based on an environment variable.

Implementation Details

• Add SECURITY_MANAGER value to AccessLevelEnum.
• Update MemberAccessLevelEnum to handle Security Manager
• Add to GitlabSubscriptions::UserRoleEnum (EE)
• Update role name transformation logic in StandardRoleType.

Acceptance Criteria

• GraphQL schema includes Security Manager enum when env var is enabled
• Schema validation passes with and without the role
• Role name transformations work correctly ("Security Manager" ↔️ "SECURITY_MANAGER")
• No GraphQL errors when role is disabled

Environment Variable

GITLAB_SECURITY_MANAGER_ROLE=true to enable

Depends on: Issue #582639