add vex tarball downloader for IoP

[pfreyburg]

Why are you introducing these changes? (Problem description, related links)

RHINENG-25683

Implement a systemd timer and service to download the VEX tarball and necessary files from the official Red Hat Product Security location (https://security.access.redhat.com/data/csaf/v2/vex/)

Key requirements:

• Read archive_latest.txt to determine the current tarball name
• Download the tarball and its .asc signature file
• Store using a stable name in the IoP environment: vex-latest.tar.zst
• Update Satellite installer (foremanctl Ansible) to generate the systemd unit files (timer and service)
• Set timer frequency to weekly, aligned with Red Hat ProdSec's update schedule

What are the changes introduced in this pull request?

• script for downloading,
• systemd service for running the script,
• systemd timer for triggering the service.

How to test this pull request

Steps to reproduce:

Enable IoP using foremanctl and then:

• vex-latest.tar.zst is downloaded into /var/www/html/pub/iop/data/csaf/v2/vex/,
• weekly timer for upcoming downloads is set.

Checklist

• Tests added/updated (if applicable)
• Documentation updated (if applicable)

[ehelms]

@pfreyburg The PR description lacks information and references a private Red Hat Jira issue.