Persist GLAS diff-based scan in rails backend

Summary

Please reference the architecture document for sast glas diff scan before starting on this for the most up-to-date context.

This issue introduces support for identifying and persisting GitLab Advanced SAST diff-based scans in the database. Specifically, if the gl-sast-report.json includes the partial_scan_mode field (added in this issue) set to differential, the scan should be marked accordingly in the Security::Scan record.

See MVC: Enable Diff-Based Scanning in MRs for Fast... (&17758 - closed) on how this issue fits in the whole plan.

Also read this thread on the discussion that led to this issue.

References

Main diff-based scanning epic: Faster Advanced SAST: Diff-based scanning in MRs (&16790 - closed)

MVC implementation epic: MVC: Enable Diff-Based Scanning in MRs for Fast... (&17758 - closed)

Implementation Plan

1. Add a new column partial_scan_mode to the security_scans table
2. Add partial scan data to the security report parser
3. Create the partial scan metadata in StoreScanService