Enable image backend and color channel format to be selectable

[vinnamkim]

Summary

• Previously we enabled the image decoding context managing only for both 3-channel colors or unchanged. However, we should extend it to image backend; and RGB or BGR color channel format selection for OTX use cases.

How to test

I updated the existing unit tests to cover this change as well.

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) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[MarkByun]

It looks good.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (eb44355) 80.53% compared to head (3324d96) 80.53%.
Report is 3 commits behind head on develop.

Files	Patch %	Lines
src/datumaro/util/image.py	89.36%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1246      +/-   ##
===========================================
- Coverage    80.53%   80.53%   -0.01%     
===========================================
  Files          270      270              
  Lines        30232    30248      +16     
  Branches      5898     5907       +9     
===========================================
+ Hits         24348    24360      +12     
- Misses        4506     4507       +1     
- Partials      1378     1381       +3     

Flag	Coverage Δ
ubuntu-20.04_Python-3.8	80.51% <89.36%> (-0.01%)	⬇️
windows-2022_Python-3.8	80.50% <89.36%> (-0.01%)	⬇️

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