Add libcudf lists column count_elements API

[davidwendt]

This adds the libcudf part of #7157

std::unique_ptr<column> cudf::lists::count_elements(
  lists_column_view const& input,
  rmm::mr::device_memory_resource* mr);

Returns the size of each element in the input lists column.
The PR also includes gtests for this new API.

[davidwendt]

rerun tests

[nvdbaranec]

Looks good to me. Couldn't hurt to add some tests that include null elements in the individual lists, even though the code isn't really affected by them

[codecov]

Codecov Report

Merging #7173 (0a04bb0) into branch-0.18 (8860baf) will increase coverage by 0.08%.
The diff coverage is 86.25%.

@@               Coverage Diff               @@
##           branch-0.18    #7173      +/-   ##
===============================================
+ Coverage        82.09%   82.17%   +0.08%     
===============================================
  Files               97       97              
  Lines            16474    16543      +69     
===============================================
+ Hits             13524    13594      +70     
+ Misses            2950     2949       -1     

Impacted Files	Coverage Δ
python/cudf/cudf/_lib/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/core/column/lists.py	91.66% <ø> (-0.09%)	⬇️
python/cudf/cudf/utils/ioutils.py	78.71% <ø> (ø)
python/cudf/cudf/core/dataframe.py	90.71% <50.00%> (+<0.01%)	⬆️
python/cudf/cudf/io/orc.py	86.80% <60.00%> (-1.61%)	⬇️
python/cudf/cudf/core/dtypes.py	89.50% <66.66%> (-0.88%)	⬇️
python/cudf/cudf/core/series.py	91.10% <80.00%> (-0.06%)	⬇️
python/cudf/cudf/core/reshape.py	91.00% <81.81%> (-0.04%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.08% <83.33%> (-0.33%)	⬇️
python/cudf/cudf/io/csv.py	91.66% <86.66%> (-1.67%)	⬇️
... and 12 more

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[devavret]

It occurred to me that list related structures are a bit of a mess. list_column_device_view's ctor is only host callable. There's an empty list_view class, yet there's a separate list_device_view class. And that despite there being a class that describes one list element, there's no element<>() accessor for it.

All of this is out of scope of this PR but just adding the latter would make this a bit proper.

[harrism]

cmake approval. Won't merge because I see there is still some discussion. @davidwendt merge when ready.

[davidwendt]

@gpucibot merge