Replace mask format support with Datumaro

[zhiltsov-max]

Added:

• Mask annotations export and import in CVAT, masks are imported as polygons
• boxes_to_masks transform
• updated documentation
• tests for Dataset

Fixed:

• Missing test for VOC source label map mode in converter
• source label map mode inserts background class as first by default
• Tests for extractors now check item subset as well

Mask format differs from the actual.
Before:

instances.zip/
├── labelmap.txt
└── SegmentationObject/
    └── image1.png

classes.zip/
├── labelmap.txt
├── SegmentationClass/
    └── image1.png

Now an archive includes both instance masks and class masks and looks like this:

archive.zip/
├── ImageSets/
│   └── Segmentation/
│       └── <subset_name>.txt
├── labelmap.txt
├── SegmentationClass/
│   └── image1.png
└── SegmentationObject/
    └── image1.png

Depends on #1140