Fix VOC mask export when background class id is not 0

[zhiltsov-max]

Summary

Related #cvat-ai/cvat#5031

• Fixes the export problem in (VOC) masks when bg label is not 0
• Refactored VOC labelmap handling code, introduced a separate class for label map
• TFDS tests are now optional depending on the package availability

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2022 CVAT.ai Corporation
#
# SPDX-License-Identifier: MIT