Add gbenchmark for converting strings to/from timestamps

[davidwendt]

Reference #5698
This creates a gbenchmark for cudf::strings::to_timestamps() and cudf::strings::from_timestamps() functions.
The benchmarks measures converting to/from a fixed format for days, seconds, ms, us, and ns.

[codecov]

Codecov Report

Merging #7641 (a97c7bf) into branch-0.19 (7871e7a) will increase coverage by 0.22%.
The diff coverage is n/a.

❗ Current head a97c7bf differs from pull request most recent head 3b0bd35. Consider uploading reports for the commit 3b0bd35 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.19    #7641      +/-   ##
===============================================
+ Coverage        81.86%   82.08%   +0.22%     
===============================================
  Files              101      101              
  Lines            16884    17036     +152     
===============================================
+ Hits             13822    13984     +162     
+ Misses            3062     3052      -10     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/categorical.py	91.62% <ø> (+0.23%)	⬆️
python/cudf/cudf/core/column/column.py	87.77% <ø> (+0.01%)	⬆️
python/cudf/cudf/core/column/datetime.py	89.09% <ø> (ø)
python/cudf/cudf/core/column/decimal.py	92.75% <ø> (-2.12%)	⬇️
python/cudf/cudf/core/column/lists.py	91.81% <ø> (+0.42%)	⬆️
python/cudf/cudf/core/column/numerical.py	94.83% <ø> (-0.20%)	⬇️
python/cudf/cudf/core/column/string.py	86.58% <ø> (+0.08%)	⬆️
python/cudf/cudf/core/column/timedelta.py	88.23% <ø> (ø)
python/cudf/cudf/core/column_accessor.py	95.28% <ø> (-0.03%)	⬇️
python/cudf/cudf/core/dataframe.py	90.79% <ø> (+0.32%)	⬆️
... and 30 more

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[codereport]

LGTM, recommend 1 small change

[davidwendt]

@gpucibot merge