extractor_tfds: add support for downloading COCO

[IRDonch]

Summary

How to test

Try to download it yourself:

$ datum download -i tfds:coco/2014 -o coco-2014

For more extreme testing, add -- --save-images. :P

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT