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ARROW-7971: [Rust] Create rowcount utility

This utility introduces a way to count the number of rows present in one or more Parquet files.  This was tested against the Parquet payloads in the `python/pyarrow/tests/data/parquet/` directory.  I created this utility out of necessity, as the `parquet-tools` project introduces applications written using Java.  This is a much faster alternative, and allows for multiple files to be counted at a time, rather than `parquet-tools` ability to only count one file at a time.

Closes #6511 from KenSuenobu/parquet-rowcount-rust and squashes the following commits:

76ce2d27e <Ken Suenobu> Renamed variable.
794237e46 <Ken Suenobu> Changed code to use row_groups() from metadata as suggested by @andygrove
31ebd3627 <Ken Suenobu> Removed formatted space.
819df7a58 <Ken Suenobu> Creation of parquet-rowcount tool to help count number of rows in a Parquet file.

Authored-by: Ken Suenobu <ksuenobu@fastmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>