Add benchmark for strings/integers convert APIs

[davidwendt]

Reference #5698
This PR adds benchmark tests for cudf::strings::to_integers and cudf::strings::from_integers. The current convert_floats_benchmark.cpp was refactored to work with floats and integer types and changed to convert_numerics_benchmark.cpp

[codecov]

Codecov Report

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[harrism]

@gpucibot merge