feat(redhat): update CSAF VEX fetcher for new feed format

[prarit]

Update the existing redhat-csaf-vex fetcher to handle the new VEX feed
format that will replace the current feed on April 1 2006.

Changes:
- Switch from zstd to gzip archive format
- Use time.Now() for archive date since the new format has no date in
  the filename, with a WithNow option for testing
- Handle both RFC3339 and colonless timezone offset formats in
  changes.csv timestamps
- Download the public repository-to-cpe.json and save all entries for
  use by the trivy-db parser
- Update test data and test infrastructure for gzip archives
- Use archive Last-Modified date instead of current time to refresh
  archive

The base URL currently points to vex-alpha for testing and will be
changed to the production vex URL after the April 1 migration.

Co-authored-by: Cursor <noreply@cursor.ai>
Signed-off-by: Prarit Bhargava <prarit@redhat.com>

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