Spike diff based scanning for advanced SAST

Summary

This issue aims to address questions raised in Faster Advanced SAST: Diff-based scanning in MRs (&16790 - closed) to support planning breakdown.

Spike tasks

• Identify code changes required to scan specific files in GLAS.
  • Discussed in this thread.
• Build a prototype and benchmark performance against a full scan
  • Discussed in this thread.
• Investigate if the selective_scan_neighborhood_depth flag is able to accurately detect a cross file vuln that spans the depth specified by the flag.
  • Discussed in this thread.
• Changes needed for the scanner report
  • Discussed in this thread.
• Investigate how scans for MRs targeting non-default branches are currently handled, and assess whether we should support this use case alongside default branch MRs.
  • Discussed in this thread
• Evaluate the SECRET_DETECTION_LOG_OPTIONS variable to determine if a similar variable is needed
  • Discussed in diff customization section of this thread
• Determine the logic for Diff calculation
  • Discussed in this thread
• Identify code changes required to display diff based scan results in the UI
  • Discussed in this thread.