Create Masks Using RectangleLabels and Export to COCO Format

**Is your feature request related to a problem? Please describe.**
I am unable to train my Mask-RCNN using bounding box annotations created in Label Studio. This is because there are no masks created when using the RectangleLabels to annotate. There is also no mask export happening when using COCO export.

**Describe the solution you'd like**
I previously used a different tool called [COCO Annotator](https://github.com/jsbroks/coco-annotator) that allowed me to annotate using bounding boxes and export annotations that had both bboxes and segmentation components of the COCO data format. Label studio exports do not have segmentation components when exporting in COCO format.

**Describe alternatives you've considered**
My alternative is at this point to switch to a Faster-RCNN that doesnt utilize masking. It should be fine but I would like to be able to do either.

**Additional context**
Annotation produced using label-studio:

Annotation produced using COCO Annotator: