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.​
• 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) 2024 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@                Coverage Diff                @@
##             releases/1.7.0    #1501   +/-   ##
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  Hits                      ?    25435           
  Misses                    ?     4639           
  Partials                  ?     1411           

Flag	Coverage Δ
ubuntu-20.04_Python-3.10	80.77% <100.00%> (?)
windows-2022_Python-3.10	80.75% <100.00%> (?)

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[wonjuleee]

Great work! Could you target this PR to releases/1.7.0?

[vinnamkim]

Rebased and targeted to releases/1.7.0