add related vulnerabilities entries for GHSA id's coming from OS entries

[westonsteimel]

What would you like to be added:

grype-db should set related vulnerabilities for GHSA entries that come from OS providers. Currently this only occurs for CVE identifiers.

Why is this needed:

The wolfi and chainguard providers can emit GHSA- identifiers from the secfixes feeds and currently there won't be a relationship set back to the corresponding identifier in the github namespaces.

This will likely end up requiring that the github provider data should be populated prior to executing the subsequent providers. I believe we already do this for nvd