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ARROW-7248: [Rust] Automatically Generate IPC Messages

This patch provides a small script that generates the code for the
Flatbuffer IPC messages automatically and performs the required
source code adjustments without manual labor.

For this first patch, it squashes the most recent changes to the Flatbuffer definitions and the generation script.

Closes #5888 from grundprinzip/master and squashes the following commits:

3ae23837e <Martin Grund> ARROW-7247:  Automatically Generate IPC Messages

Authored-by: Martin Grund <magrund@amazon.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>