Add .list.astype() to cast list leaves to specified dtype

[shwina]

Closes #10687.

[codecov]

Codecov Report

Merging #10693 (abc1dc8) into branch-22.06 (65b1cbd) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head abc1dc8 differs from pull request most recent head 53ca862. Consider uploading reports for the commit 53ca862 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.06   #10693      +/-   ##
================================================
+ Coverage         86.35%   86.38%   +0.03%     
================================================
  Files               142      142              
  Lines             22335    22339       +4     
================================================
+ Hits              19287    19298      +11     
+ Misses             3048     3041       -7     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/lists.py	92.91% <100.00%> (+1.39%)	⬆️
python/cudf/cudf/core/column/string.py	89.22% <0.00%> (+0.12%)	⬆️
python/cudf/cudf/core/groupby/groupby.py	91.64% <0.00%> (+0.23%)	⬆️
python/cudf/cudf/core/column/numerical.py	96.17% <0.00%> (+0.29%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.49% <0.00%> (+0.30%)	⬆️

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[rgsl888prabhu]

A small query

[shwina]

@gpucibot merge