Add image sha256 to tfrecord

[zhiltsov-max]

Motivation and context

Closes #1829. It looks like this field is not mandatory.

Added image/sha256 field to tfrecords.

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 6554

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 64.991%

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Totals
Change from base Build 6548:	0.02%
Covered Lines:	11067
Relevant Lines:	16622

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💛 - Coveralls

[nmanovic]

@zhiltsov-max , please add a line into CHANGELOG.md. It seems I have merged the previous issue without a line in CHANGELOG.md. Probably you want to fix that in the PR.