Add benchmark for strings/fixed_point convert APIs

[davidwendt]

Reference #5698
This PR adds benchmark tests for cudf::strings::to_fixed_point and cudf::strings::from_fixed_point.
The benchmarks include measurements for both decimal32 and decimal64 types.

[codereport]

lgtm, other than the two comments (maybe it isn't possible)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.08@48a2663). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             branch-21.08    #8417   +/-   ##
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  Coverage                ?   82.83%           
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  Files                   ?      109           
  Lines                   ?    17901           
  Branches                ?        0           
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  Hits                    ?    14828           
  Misses                  ?     3073           
  Partials                ?        0           

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[harrism]

@gpucibot merge