Add random_posterization processing layer

[shashaka]

I implemented the RandomPosterization image processing feature before working on rand_augment, as it includes related image processing functionalities.

Here is my gist

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.77778% with 14 lines in your changes missing coverage. Please review.

Project coverage is 81.95%. Comparing base (f54c127) to head (1cc6026).

Files with missing lines	Patch %	Lines
...essing/image_preprocessing/random_posterization.py	76.66%	8 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20688      +/-   ##
==========================================
- Coverage   81.96%   81.95%   -0.01%     
==========================================
  Files         545      546       +1     
  Lines       50831    50894      +63     
  Branches     7861     7869       +8     
==========================================
+ Hits        41663    41712      +49     
- Misses       7265     7273       +8     
- Partials     1903     1909       +6     

Flag	Coverage Δ
keras	81.79% <77.77%> (-0.01%)	⬇️
keras-jax	64.00% <77.77%> (+0.04%)	⬆️
keras-numpy	58.92% <71.42%> (+0.03%)	⬆️
keras-openvino	29.93% <31.74%> (+<0.01%)	⬆️
keras-tensorflow	64.72% <77.77%> (+0.01%)	⬆️
keras-torch	63.85% <77.77%> (+0.04%)	⬆️

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