[Rust] Create build.rs to generate flatbuffers files

[asfimport]

We should take the logic from the regen.sh [1] bash script and convert it into a Rust build.rs script that can run in CI. This would require flatc to be installed to be able to build the project.

 

[1] https://github.com/apache/arrow/blob/master/rust/arrow/regen.sh

Reporter: Andy Grove / @andygrove

_{Note: This issue was originally created as ARROW-7642. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Migrated to github: apache/arrow-rs#59