[REVIEW] Add dictionary support to cudf::minmax

[davidwendt]

Reference #5963

Add support for dictionary column types in cudf::minmax. This PR also adds support for strings column types as well. The code was refactored to re-use the internal reduce_device function for all supported types. The strings specialization logic just creates a string_scalar result which requires a copy step to build a new string_scalar object from a string_view (output from the reduce_device function). The dictionary support was to just to substitute the keys() column in the detail function right before the type-dispatcher call.

Added a gtests for strings as well as dictionary with strings keys.

[codecov]

Codecov Report

Merging #6764 (cc8e21d) into branch-0.17 (8c420ae) will increase coverage by 0.71%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.17    #6764      +/-   ##
===============================================
+ Coverage        81.71%   82.42%   +0.71%     
===============================================
  Files               96       96              
  Lines            15978    16843     +865     
===============================================
+ Hits             13056    13883     +827     
- Misses            2922     2960      +38     

Impacted Files	Coverage Δ
python/cudf/cudf/_fuzz_testing/fuzzer.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/hash_vocab_utils.py	100.00% <0.00%> (ø)
python/cudf/cudf/core/column/column.py	87.70% <0.00%> (+0.10%)	⬆️
python/cudf/cudf/core/series.py	92.80% <0.00%> (+1.54%)	⬆️
python/cudf/cudf/utils/utils.py	87.82% <0.00%> (+3.93%)	⬆️
python/cudf/cudf/core/abc.py	91.48% <0.00%> (+4.25%)	⬆️
python/cudf/cudf/utils/gpu_utils.py	58.53% <0.00%> (+4.87%)	⬆️

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[devavret]

Just realized something. Not requesting changes because need confirmation if what I'm thinking is correct.

Caught something after the review

[devavret]

One non-dealbreaker comment but approve otherwise.