Output EPSS as rating in VEX

### Current Behavior

VEX output contains only CVSS rating.

No export feature on "Exploit Predictions" tags.

To export EPSS, you need to parse `finding/project/{uuid}` API response.

### Proposed Behavior

Add a second `ratings` in VEX export including EPSS score

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/main/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

Trace the VEX export path and compare its current CVSS rating data with the finding/project/{uuid} API response, which exposes EPSS. Confirm how VEX ratings are represented and identify the relevant export tests before changing anything. Done means VEX output includes a second rating for the EPSS score and existing CVSS output remains intact.

Written by the indexing model from the issue text.