[REVIEW] Fix StructColumn.to_pandas type handling issues

[galipremsagar]

Fixes: #9383

This PR preserves datetime & timedelta types in StructColumn upon conversion to pd.Series, and also preserves int values to be int rather than converting to float.

[codecov]

Codecov Report

Merging #9388 (96327e5) into branch-21.12 (ab4bfaa) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

❗ Current head 96327e5 differs from pull request most recent head cb299b4. Consider uploading reports for the commit cb299b4 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.12    #9388      +/-   ##
================================================
- Coverage         10.79%   10.74%   -0.05%     
================================================
  Files               116      116              
  Lines             18869    19086     +217     
================================================
+ Hits               2036     2051      +15     
- Misses            16833    17035     +202     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_lib/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/_base_index.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/categorical.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/column.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/datetime.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/lists.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/numerical.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/string.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/struct.py	0.00% <0.00%> (ø)
... and 26 more

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[galipremsagar]

@gpucibot merge