[C++] include-what-you-use reports

[Wes McKinney <we...@gmail.com>]

I occasionally run the IWYU report (docker-compose run iwyu) to clean
up includes and I notice that the codebase could really use some love
in this department

https://gist.github.com/wesm/af59c7cc8f35c6fd806b0d041b816da8

Some of our IWYU filter settings are a little bit off, so I don't
think everything can be 100% fixed, but there's a lot of stuff that
would be good to clean up. I'm going to clean up all the
Parquet-related includes as part of a patch I'm working on, but I
encourage other C++ developers to make this report part of your
development workflow to keep things tidy. Someday when we have more
maintainer bandwidth we might introduce this as part of our CI
(nightly or otherwise) but I don't think we can justify the effort
right now.

- Wes