Support import of OpenVEX statements

### Current Behavior

During the work on #7076, it became clear that the VEX ecosystem is not converging on any one standard, but is instead fragmented. Many CLI tools, while offering CycloneDX VEX support, default or recommend [OpenVEX](https://openssf.org/projects/openvex/).

We're missing out on real interop-opportunities by not supporting OpenVEX. An eventual integration of CycloneDX Transparency Exchange API (TEA) will amplify this gap.

### Proposed Behavior

Add the ability to import OpenVEX statements. Export is out of scope for this issue and may be requested separately.

### 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

Start by reviewing the existing CycloneDX VEX import support and the OpenVEX specification. Completion means OpenVEX statements can be imported successfully; export is explicitly out of scope.

Written by the indexing model from the issue text.