Refactor pad/zfill functions for reuse with strings udf

[davidwendt]

Description

Refactors the main device code used for cudf::strings::pad and cudf::strings::zfill for reuse in strings UDF pad and zfill functions. No new functions or features have been added, updated, or removed. The detail functions have been mainly just be moved to new file cpp/include/cudf/strings/detail/pad_impl.cuh

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

Base: 87.40% // Head: 88.12% // Increases project coverage by +0.71% 🎉

Coverage data is based on head (f6e8da0) compared to base (f72c4ce).
Patch coverage: 88.48% of modified lines in pull request are covered.

❗ Current head f6e8da0 differs from pull request most recent head 148f7b4. Consider uploading reports for the commit 148f7b4 to get more accurate results

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #11914      +/-   ##
================================================
+ Coverage         87.40%   88.12%   +0.71%     
================================================
  Files               133      133              
  Lines             21833    21905      +72     
================================================
+ Hits              19084    19304     +220     
+ Misses             2749     2601     -148     

Impacted Files	Coverage Δ
python/cudf/cudf/core/udf/__init__.py	97.05% <ø> (+47.05%)	⬆️
python/cudf/cudf/io/orc.py	92.94% <ø> (-0.09%)	⬇️
python/cudf/cudf/testing/dataset_generator.py	72.83% <ø> (-0.42%)	⬇️
...thon/dask_cudf/dask_cudf/tests/test_distributed.py	18.86% <ø> (+4.94%)	⬆️
python/cudf/cudf/core/_base_index.py	82.20% <43.75%> (-3.35%)	⬇️
python/cudf/cudf/io/text.py	91.66% <66.66%> (-8.34%)	⬇️
python/strings_udf/strings_udf/__init__.py	84.31% <76.00%> (-12.57%)	⬇️
python/cudf/cudf/core/index.py	92.96% <95.71%> (+0.33%)	⬆️
python/cudf/cudf/__init__.py	90.69% <100.00%> (ø)
python/cudf/cudf/_lib/strings/__init__.py	100.00% <100.00%> (ø)
... and 17 more

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[upsj]

LGTM!

[davidwendt]

@gpucibot merge