Add possibility to export Policy Clearing

### Current Behavior

Currently, after clearing policy violations and then creating a new version of a project, all violations have to be cleared again.

### Proposed Behavior

It would be nice to be able to export the cleared policy violations (similar to the VEX export on vulnerabilities), so we don't have to clear all policies again every time a new version of a project is created.

### 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