Add artifacts dropdown to security MR widget

What does this MR do?

Add artifacts dropdown to security MR widget

This implements the report artifacts download dropdown on the Core security MR widget, for #249544 (closed).

This is implemented behind the core_security_mr_widget_downloads feature flag, whose rollout is tracked by #273418 (closed).

Screenshots (strongly suggested)

Feature flag statuses	coreSecurityMrWidgetCounts ❌	coreSecurityMrWidgetCounts ✅
coreSecurityMrWidgetDownloads ❌	i.e., no visual change from current master.	i.e., no visual change from what was implemented in !47656 (merged). To recreate this screenshot, follow the testing instructions in that MR.
coreSecurityMrWidgetDownloads ✅

Testing locally

• To recreate the screenshots with vulnerability counts, follow the instructions in !47656 (merged).
• To recreate the screenshots with the download dropdown, enable the feature flag with:

  echo "::Feature.enable(:core_security_mr_widget_downloads)" | bundle exec rails console

TODO

Verify downloads dropdown works in these plans:

• Core (FOSS)
• EE
  • Free
  • Bronze
  • Silver

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #273423 (closed)