Enhance lib/github_advisory_sync.rb script

[jasnow]

Change lib/github_advisory_sync.rb script to add a field and value to existing advisories if field is currently absent.

1. Any advisories without "patched_versions:" (102)
2. Any advisories without "cve:" (128)
3. Any advisories without "ghsa:" (66)

The above number in ()s are the existing advisories without those fields.

Suggest doing the above fields one at a time so to make PR changes easier.