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.*}

该 issue 未指定任何文件或测试；请先定位 repository 的 CI 和 build 配置，然后确定 FindBugs 支持及其文档应放置的位置。完成标准是 CI 运行 FindBugs 作为参考而不使 build 失败，并且文档说明之后如何强制执行警告或错误。

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