[Datumaro] Fixes

[zhiltsov-max]

Motivation and context

• Bboxes in COCO format no more converted to polygons on exporting. This allows to better keep original annotations.
• REST API extractor produces images with absolute paths to its cache.
• Datumaro format keeps input image paths (including absolute ones).
• Fixed definition in datumaro format tests.

How has this been tested?

Unit tests

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6631

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.003%) to 65.015%

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Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	1	77.29%

Totals
Change from base Build 6627:	-0.003%
Covered Lines:	11104
Relevant Lines:	16676

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💛 - Coveralls