add script to regenerate x509 test fixtures

[jcharaoui]

Short description

This adds a Ruby script that handles regenerating x509 test fixtures.

It has been carried by the downstream Debian package for over a year and is considered battle-tested by Debian's own CI and package-rebuild infrastructure.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[jcharaoui]

I Just noticed that 0a50e51 makes this irrelevant, closing.

[jcharaoui]

Actually I'm not so sure this should be discarded, because we still need a way to regenerate these fixtures programatically. Regenerating them once every 5 years doesn't work well for longer lived packages published in OS release like Debian's.