refactor: replace static methods in some transformers

[premsrii]

This PR replaces some of the inner static methods that are used with pl.DataFrame.apply(), with suitable alternatives. The purpose is to improve the pytest code coverage, which currently ignores these functions.

[codecov]

Codecov Report

Merging #110 (a3e8121) into develop (c493677) will decrease coverage by 3.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #110      +/-   ##
===========================================
- Coverage    99.42%   96.40%   -3.02%     
===========================================
  Files            8        8              
  Lines          519      501      -18     
===========================================
- Hits           516      483      -33     
- Misses           3       18      +15     

Impacted Files	Coverage Δ
src/sk_transformers/__init__.py	100.00% <100.00%> (ø)
src/sk_transformers/datetime_transformer.py	100.00% <100.00%> (ø)
src/sk_transformers/generic_transformer.py	100.00% <100.00%> (+0.52%)	⬆️
src/sk_transformers/number_transformer.py	100.00% <100.00%> (ø)
src/sk_transformers/string_transformer.py	84.07% <100.00%> (-14.42%)	⬇️
src/sk_transformers/utils.py	100.00% <100.00%> (ø)

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