Add factories for advisory fields

Summary

This is a follow-up issue that was originally discussed in !117326 (comment 1367501082).

Improvements

• Add a factory for CVE and Gemnasium identifiers.
• Optional Add a CVSS factory or sequence

Risks

No risks have been identified at the moment.

Involved components

ee/spec/factories/package_metadata/pm_advisories.rb

Optional: Intended side effects

Identifiers and possibly CVSS strings are dynamically generated.

Optional: Missing test coverage

N/A

Implementation Plan

• Add a CVE identifier factory that generates a valid identifier object. See the schema introduced for this in !117326 (merged)
• Add a Gemnasium identifier factory that generates a valid identifier object. See the schema introduced for this in !117326 (merged)