Refactor libcudf strings concatenate to use make_strings_children

[davidwendt]

Refactor combine.cu to split out join_strings() and enable concatenate() to use make_strings_children utility.

[davidwendt]

Reference #7929

[codecov]

Codecov Report

Merging #7937 (106c070) into branch-0.20 (599f62d) will increase coverage by 0.41%.
The diff coverage is 92.73%.

❗ Current head 106c070 differs from pull request most recent head 0615000. Consider uploading reports for the commit 0615000 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.20    #7937      +/-   ##
===============================================
+ Coverage        82.30%   82.71%   +0.41%     
===============================================
  Files              101      103       +2     
  Lines            17053    17711     +658     
===============================================
+ Hits             14035    14650     +615     
- Misses            3018     3061      +43     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/dtypes.py	83.44% <ø> (-6.45%)	⬇️
python/cudf/cudf/utils/utils.py	83.25% <ø> (-1.81%)	⬇️
python/cudf/cudf/core/groupby/groupby.py	92.41% <78.57%> (-1.04%)	⬇️
python/cudf/cudf/core/column/lists.py	87.41% <80.00%> (+0.19%)	⬆️
python/cudf/cudf/core/column/struct.py	96.29% <86.66%> (-3.71%)	⬇️
python/cudf/cudf/core/index.py	93.04% <88.09%> (+0.01%)	⬆️
python/cudf/cudf/core/column/column.py	87.86% <88.57%> (+0.43%)	⬆️
python/cudf/cudf/core/column/decimal.py	92.92% <91.48%> (-0.92%)	⬇️
python/cudf/cudf/core/column/interval.py	91.11% <92.30%> (+0.48%)	⬆️
python/cudf/cudf/core/join/join.py	96.75% <93.75%> (+0.24%)	⬆️
... and 68 more

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[davidwendt]

@gpucibot merge