Add findbugs support, esp for @Nullable / @NonNull annotation

It'd be really great to have our CI run findbugs. This should document how to fail the build if there are any warnings or errors, but not turn it on yet (though it should still run with CI so we can reference it).

It would be great to have the build enforce using Preconditions and null checks, among other benefits of static analysis

**Reporter**: [Jonathan Coveney](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=jcoveney) / @jcoveney

_{**Note**: *This issue was originally created as [PARQUET-60](https://issues.apache.org/jira/browse/PARQUET-60). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}