Improve MR Experience for Secure Scans on Non-Ultimate Plans - Add artifact download dropdown

This issue tracks the addition of an artifact download dropdown in the security MR widget for non-Ultimate users. It relies on backend work in #251015 (closed).

Implementation Plan

• Implement behind core_security_mr_widget_downloads feature flag, disabled by default
• Implement frontend GraphQL query to fetch job artifact details
• Add GlDropdown to main component added by #249543 (closed), and feed results of GraphQL query into it