Add CVSS score for GHSA vulnerabilites

### Current Behavior

Currently, all vulnerabilities in the GitHub Security Advisories database do not have a CVSS score, even though they are available on the GitHub page.
For instance, `CVE-2019-10086` has a CVSS score of 7.3 in the DTrack database, but the alias `GHSA-6phf-73q6-gh87` does not have any CVSS score. However, if we visit the vulnerability page on GitHub (https://github.com/advisories/GHSA-6phf-73q6-gh87), we can see that there is a CVSS score with a detailed vector that can be loaded and used by Dependency Track.

### Proposed Behavior

It's needed to download and utilize the CVSS score from the GHSA database to ensure a more comprehensive vulnerability database and make informed decisions regarding the vulnerabilities found

### Checklist

- [X] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [X] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested