Support Grafeas API

Grafeas is an attempt to consolidate findings of many sources in a common standard format.
https://cloud.google.com/blog/products/gcp/introducing-grafeas-open-source-api-
https://grafeas.io/

### Current Behavior:
Findings can only be handled within dependencytrack.

### Proposed Behavior:
Provide an API that supports using dependency-track findings together with findings from other sources in a standard way like grafeas.