Add support for Gitlab Advisory Database

### Current Behavior:
Dependency Track has support for various vulnerability / advisory databases. Adding more could improve the accuracy, as especially Sonatype OSS index is quite slow to fix missing/wrong advisories.

### Proposed Behavior:
Add support for Gitlab advisory database, as can be found on https://gitlab.com/gitlab-org/security-products/gemnasium-db/-/tree/master/