feat(vex): integrate lib4vex for VEX document management

[JigyasuRajput]

Integrate lib4vex for VEX document handling

This PR introduces a centralized VexHandler class that leverages lib4vex for parsing, validation, generation, and conversion of VEX documents across multiple formats (CSAF, CycloneDX, and OpenVEX).

Changes

• Created VexHandler class to provide a unified interface for all VEX operations
• Refactored VEX module to use lib4vex APIs instead of manual JSON handling
• Added comprehensive unit tests for the new functionality
• Implemented support for multiple VEX formats with consistent handling

Testing

Unit tests cover:

• Parsing different VEX formats
• Validation of VEX documents
• Generation of VEX documents
• Format conversion between different VEX types

It is a draft PR and the code still needs changes before merging it

[anthonyharrison]

Looking good.

Be careful using logger.info as this can become very noisey very quickly. Suggest that you change this to debug.

[JigyasuRajput]

Hey! I've implemented the suggested changes and fixed the tests for Week 1 tasks. However, CI on my fork is acting up a bit (even after a re-run) ...waiting for the CI here. Let me know if there's any changes required.
Thanks!
cc @anthonyharrison @terriko

[terriko]

I've approved CI to run, and I've also submitted a ticket to get you added to the "cve-bin-tool-read" group so you shouldn't have to wait for me to trigger CI runs. You'll get an invite in the next day or so when our ops team gets time to send them out, please accept it!

[mastersans]

Looks good to me, Some Vex and Vex Handler Tests are failing, I'll make another pass once they are fixed.