Add gbenchmark for cudf::strings::to_lower

[davidwendt]

Reference #5698
This creates a gbenchmark for the cudf::strings::to_lower. The device logic is the same for cudf::strings::to_upper and cudf::strings::swapcase so this a good measure for the 3 APIs.

This PR is dependent on changes in PR #7292
These are mostly in the generate_benchmark_input.cpp

The initial results were as follows:

--------------------------------------------------------------------------------------------------
Benchmark                                        Time             CPU   Iterations UserCounters...
--------------------------------------------------------------------------------------------------
StringCase/strings/4096/manual_time          0.278 ms        0.296 ms         2514 bytes_per_second=248.756M/s
StringCase/strings/32768/manual_time         0.289 ms        0.307 ms         2421 bytes_per_second=1.86625G/s
StringCase/strings/262144/manual_time        0.419 ms        0.438 ms         1662 bytes_per_second=10.2869G/s
StringCase/strings/2097152/manual_time        2.59 ms         2.61 ms          269 bytes_per_second=13.3449G/s
StringCase/strings/16777216/manual_time       25.9 ms         25.9 ms           27 bytes_per_second=10.6531G/s

The convert_case code here is a bit old. I changed it to use the more efficient make_strings_children utility and found the performance improved by 2x

--------------------------------------------------------------------------------------------------
Benchmark                                        Time             CPU   Iterations UserCounters...
--------------------------------------------------------------------------------------------------
StringCase/strings/4096/manual_time          0.117 ms        0.135 ms         5877 bytes_per_second=592.795M/s
StringCase/strings/32768/manual_time         0.122 ms        0.140 ms         5641 bytes_per_second=4.42664G/s
StringCase/strings/262144/manual_time        0.274 ms        0.292 ms         2535 bytes_per_second=15.768G/s
StringCase/strings/2097152/manual_time        1.59 ms         1.61 ms          441 bytes_per_second=21.759G/s
StringCase/strings/16777216/manual_time       12.1 ms         12.1 ms           58 bytes_per_second=22.8626G/s

So these changes are also included in this PR.

[codereport]

looks good, couple small things

[codecov]

Codecov Report

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[davidwendt]

rerun tests

[davidwendt]

rerun tests

[harrism]

cmake approval

[davidwendt]

@gpucibot merge