pre annotation (importing pascal VOC format (xml))

**Is your feature request related to a problem? Please describe.**
Specifically looking this feature for **BoundingBox**
we want to load the images and their corresponding annotation(pascal voc format) while launching the labelstudio. we want to know whether labelstudio supports importing pascal voc rather than completions json format or is there any converter available to convert pascal voc to completion json.

**Describe the solution you'd like**

- load images, config and their corresponding (pascal voc)annotation (if available)
- if the annotation is ok then proceed with pre annotated bbox else they can modify the bounding box