[REVIEW] Enable basic reductions for decimal columns

[ChrisJar]

Enables sum, min, max, product, and sum_of_squares for DecimalColumn

Closes #7683

[davidwendt]

This should probably re-targeted 0.20?
@kkraus14

[kkraus14]

This should probably re-targeted 0.20?
@kkraus14

No this is planned for 0.19.

[harrism]

Lots of things were planned for 0.19 that we would reject if the PR was opened during burndown...

[codecov]

Codecov Report

Merging #7776 (85dbadd) into branch-0.19 (7871e7a) will increase coverage by 0.78%.
The diff coverage is n/a.

❗ Current head 85dbadd differs from pull request most recent head d31535b. Consider uploading reports for the commit d31535b to get more accurate results

@@               Coverage Diff               @@
##           branch-0.19    #7776      +/-   ##
===============================================
+ Coverage        81.86%   82.65%   +0.78%     
===============================================
  Files              101      103       +2     
  Lines            16884    17578     +694     
===============================================
+ Hits             13822    14529     +707     
+ Misses            3062     3049      -13     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/dtypes.py	83.44% <0.00%> (-6.08%)	⬇️
python/cudf/cudf/core/column/lists.py	87.32% <0.00%> (-4.08%)	⬇️
python/cudf/cudf/core/column/decimal.py	92.55% <0.00%> (-2.32%)	⬇️
python/dask_cudf/dask_cudf/backends.py	87.50% <0.00%> (-2.13%)	⬇️
python/cudf/cudf/core/groupby/groupby.py	92.41% <0.00%> (-1.04%)	⬇️
python/cudf/cudf/core/column/column.py	87.61% <0.00%> (-0.15%)	⬇️
python/cudf/cudf/utils/utils.py	85.36% <0.00%> (-0.07%)	⬇️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/ioutils.py	78.71% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
... and 46 more

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[isVoid]

lgtm!

[kkraus14]

@gpucibot merge