Add support for instance segmentation

[gdlg]

This PR adds a new mask type for instance segmentation. The difference with semantic segmentation is that the semantic segmentation mask is a 2D mask containing the class ID associated with each pixel while the instance segmentation mask is a stack of boolean masks, one for each object.

The PR adds the following:

• Converter from polygons to masks
• Converter from polygons to bounding boxes because many instance segmentation algorithm needs both the masks and the bounding box of each object

Checklist

• I have added tests to cover my changes or documented any manual tests.
• I have added the description of my changes into CHANGELOG.
• I have updated the documentation accordingly

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 52.56410% with 37 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/datumaro/experimental/converters.py	53.22%	29 Missing ⚠️
src/datumaro/experimental/fields.py	50.00%	8 Missing ⚠️

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