feat(ghsa): use nuget advisories in lower case

[DmitriyLewen]

Description

This PR implements case-insensitive storage for NuGet packages in the vulnerability database while maintaining backward compatibility. The changes include:

• NuGet Package Normalization: Modified NormalizePkgName in pkg/vulnsrc/vulnerability/vulnerability.go:261 to convert NuGet package names to lowercase, making them case-insensitive as per NuGet specification
• Duplicate Advisory Storage: Enhanced TransformAdvisories in pkg/vulnsrc/ghsa/ghsa.go:143 to store both lowercase and original-case versions of NuGet advisories for backward compatibility
• Comprehensive Test Coverage: Added test case in pkg/vulnsrc/ghsa/ghsa_test.go:446 with example NuGet advisory data to verify the dual storage functionality

The implementation stores duplicate entries during a transition period - one with the normalized (lowercase) package name and another with the original case to ensure existing downstream consumers continue to work.

Reasons for Change

1. NuGet Case-Insensitivity Compliance: NuGet package names are case-insensitive by design, but the current implementation was storing them with their original case, leading to potential inconsistencies and missed vulnerability matches
2. Backward Compatibility Requirements: Existing downstream tools and integrations may rely on the original case format, so we need to maintain both versions during a migration period
3. Standardization: This change aligns the vulnerability database with NuGet's official case-insensitive behavior, improving accuracy and consistency of vulnerability detection
4. Future Migration Path: The TODO comment indicates this is a transitional approach, with plans to eventually remove the original-case entries once downstream consumers migrate to the lowercase format

DB changes

buckets:

• before:
  
• after:
  

sizes:

• before:

   ➜  du -sh ./assets-original/trivy.db 
   715M    ./assets-original/trivy.db

• after:

   ➜  du -sh ./assets/trivy.db
   716M    ./assets/trivy.db

[knqyf263]

nit: We may want to use switch here.

switch adv.Ecosystem { 
  case vulnerability.Swift:
  case vulnerability.Go:
  case vulnerability.NuGet:

}

[DmitriyLewen]

Make sense. Updated in 5bc3cdf