Support diff-based scanning in security-report-schema

Summary

Update the security report schemas to support a new optional partial_scan_mode enum field with the value differential for GLAS diff-based scans.

• Since not all analyzers support partial scan modes, this field should be optional. Reports that don't include it can be assumed to have a null value.
• When incremental scanning is introduced, a new enum value of incremental can be added.

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

Also read this thread for context on the changes needed here.

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 the partial_scan_mode field to the security report schema
   1. Field should be of type enum
   2. It should be optional
   3. Support differential as an enum value