Add CocoRoboflowImporter

[vinnamkim]

Summary

• Ticket no. 109027
• From 'otx train' in workspace throws (dataset?) error training_extensions#2042, we found that there is a demand to import a COCO format dataset provided by Roboflow. Currently, Datumaro cannot import it because they have different directory structure.
• Did some refactoring for the existing data_format tests

How to test

I added tests for this change.

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

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) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Patch coverage: 96.36% and project coverage change: +0.01 🎉

Comparison is base (769c620) 78.70% compared to head (380dcad) 78.72%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #976      +/-   ##
===========================================
+ Coverage    78.70%   78.72%   +0.01%     
===========================================
  Files          231      231              
  Lines        26420    26463      +43     
  Branches      5258     5263       +5     
===========================================
+ Hits         20795    20834      +39     
- Misses        4400     4402       +2     
- Partials      1225     1227       +2     

Flag	Coverage Δ
macos-11_Python-3.8	77.76% <96.36%> (+0.03%)	⬆️
ubuntu-20.04_Python-3.8	78.72% <96.36%> (+0.03%)	⬆️
windows-2019_Python-3.8	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datumaro/plugins/data_formats/coco/base.py	92.30% <93.33%> (+0.21%)	⬆️
datumaro/plugins/data_formats/coco/format.py	100.00% <100.00%> (ø)
datumaro/plugins/data_formats/coco/importer.py	95.37% <100.00%> (+1.05%)	⬆️

... and 1 file with indirect coverage changes

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