Enhancement request: Export OpenVEX VEX docs

[pombredanne]

Is your enhancement request related to a problem? Please describe.
Describe the problem that you would like to address.

Like for CSAF and CDX, OpenVEx is an alternative document format for VEX, all PURL-based
https://github.com/openvex/spec

What are the benefits of the requested enhancement?
How will the new functionality benefit DejaCode users?

It helps folks that use this format like Ubuntu

• https://github.com/canonical/ubuntu-security-notices/tree/main/vex
• https://github.com/canonical/ubuntu-security-notices/blob/main/vex/usn/USN-2169-1.json

Describe the solution you would like
Provide a concise scenario or use case that needs to be supported in DejaCode.

An export the same way we have CSAF and CDX VEX export.

Additional notes
Add any other context or screenshots about the feature request here.

Some implementation

• https://github.com/anthonyharrison/lib4sbom

[tdruez]

OpenVEX export Implemented in #442