Add correct URL per identifier

What does this MR do?

• Updates identifiers.go with ParseIdentifiers and related functions from reports/v3
• Updates reports_test.go
• Adds a unit test for ConvertIdentifiers
• Updates converter_test.go base64 expected output

Why are we doing this?

In order to return a report that contains the correct identifier URLs for each vulnerability. Until now we always returned an nvd kind of URL. For non CVEs the URL was returning 404.

What are the relevant issue numbers?

Return the correct Identifier URL for OCS vulne... (gitlab-org/gitlab#469241 - closed)