Improve PIL and COLOR_BGR context image decode performance

[vinnamkim]

Summary

• Ticket no. 141409
• There was a significant performance degradation on decode_by_pil() if ImageColorChannel.COLOR_BGR.
• Improve it by returning a numpy array rather than PIL.Image and using cv2.cvtColor() rather than np.flip().

How to test

• tests/unit/test_image.py covers this change.
• Compare performance with this code (dataset: Imagenette2)

before	after
00:51<00:00, 361.65it/s	00:19<00:00, 859.39it/s

Checklist

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

License

• [x] 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.
• [x] I have updated the license header for each file (see an example below).

# Copyright (C) 2024 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## releases/1.7.0 #1501 +/- ## ================================================= Coverage ? 80.78% ================================================= Files ? 276 Lines ? 31485 Branches ? 6345 ================================================= Hits ? 25435 Misses ? 4639 Partials ? 1411 ``` | [Flag](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1501/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | Coverage Δ | | |---|---|---| | [ubuntu-20.04_Python-3.10](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1501/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | `80.77% <100.00%> (?)` | | | [windows-2022_Python-3.10](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1501/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | `80.75% <100.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit#carryforward-flags-in-the-pull-request-comment) to find out more.

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[vinnamkim]

Rebased and targeted to releases/1.7.0