[Pre537] Add cvss values back in as needed using external data in github_advisory_sync.rb

[jasnow]

Add cvss (cvss_v2, cvss_v3, cvss_v4) values back in as needed using external data in github_advisory_sync.rb.

• Rest API Example: https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2019-1010218

[postmodern]

Note that NVD aggressively rate limits their REST API access.

[jasnow]

Seems this topic was discussed in the past: #399