[Datumaro] Allow any annotations file name in coco extractor

[zhiltsov-max]

Motivation and context

If we add a file directly by its path, there is no reason to disallow arbitrary file names. The name should be used to recognize subset, but if it can't be done we can just use the default subset. This does not happen in official COCO format, but for custom annotation this improves quality of life.

How has this been tested?

Manual test

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 5493

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.04%) to 65.643%

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Files with Coverage Reduction	New Missed Lines	%
datumaro/datumaro/components/extractor.py	1	90.59%
cvat/apps/engine/media_extractors.py	8	75.29%

Totals
Change from base Build 5489:	-0.04%
Covered Lines:	10887
Relevant Lines:	16181

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💛 - Coveralls