Add API `GET /v1/vex/token/{uuid}` to query state of VEX processing

### Current Behavior

When uploading a VEX file through REST API `POST /api/v1/vex`, the VEX file is being processed, which takes some time.
However there is no capability to query if VEX processing was finished or if it's still ongoing.

For subsequent API calls, I want to make sure that VEX processing is finished.

### Proposed Behavior

Please add capabilities to return a token from `POST /api/v1/vex` which can be used with `GET /v1/vex/token/{uuid}` to query the state of VEX processing - similar to BOM processing `PUT /api/v1/bom` and `GET /v1/bom/token/{uuid}`

### 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