Add support for github purl-type

[gwnlng]

This is a suggestion to add support for github purl-type at snyk sbom enrichment of security vulnerabilities details.

[garethr]

Good suggestion. For this, we'd need to index our vulnerabilities by the GitHUb source, which we don't currently do. And then expose that in the API. It might be possible to use the ecosystems data to go from a github purl to one that references a package manager however.