[REVIEW] Add dictionary support to cudf::clamp

[davidwendt]

Reference #5963

Add specialization logic for dictionary type to the cudf::clamp API.
This required introducing a cudf::dictionary::detail::get_insert_index function to retrieve an appropriate scalar index from an input scalar key value.
This PR includes gtests for both the dictionary specialization and the new get_insert_index function.

[codecov]

Codecov Report

Merging #6373 into branch-0.16 will increase coverage by 0.33%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.16    #6373      +/-   ##
===============================================
+ Coverage        82.82%   83.15%   +0.33%     
===============================================
  Files               94       94              
  Lines            14432    14779     +347     
===============================================
+ Hits             11953    12290     +337     
- Misses            2479     2489      +10     

Impacted Files	Coverage Δ
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/testing/io.py	0.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/custreamz/custreamz/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_csv.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_orc.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_json.py	100.00% <0.00%> (ø)
...ython/custreamz/custreamz/tests/test_dataframes.py	100.00% <0.00%> (ø)
...ython/dask_cudf/dask_cudf/io/tests/test_parquet.py	100.00% <0.00%> (ø)
... and 34 more

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[harrism]

Looks good. Just DRYing

[davidwendt]

rerun tests

[trxcllnt]

Left a question on dispatch_clamp null handling.