Move strings/numeric convert benchmarks to nvbench

[davidwendt]

Description

Moves the cpp/benchmarks/string/convert_numerics.cpp and cpp/benchmarks/string/convert_fixed_point.cpp benchmark implementations from google-bench to nvbench.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

/merge