Moves container scanning logic to the backend

What does this MR do?

This moves the reports logic from the frontend to the backend for container scanning reports in on an MR. It's behind the feature flag containerScanningMergeRequestReportApi which is disabled currently.

Once the feature flag is enabled, the user will see no changes. We generate the same report, just in the backend instead of the frontend. This should improve performance, load times, and code reuse.

If you need it, here's an explainer video: https://youtu.be/WzgsWDcvmes

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance 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

Closes #13518 (closed)