[Java] Include compiled Flatbuffers sources in release artifacts

[asfimport]

I'm seeing the flatc that's available in Maven foul up users because it depends on a version of glibc (2.14) which is too new for many enterprise Linux users. It would be better for the Arrow Java artifacts to not depend on flatc at all, if possible.

We could check the compiled sources into source control, but that seems less than ideal. Is there a clean way bundle them at packaging time?

Reporter: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-397. Please see the migration documentation for further details.}