[Java] Provide a sample json file for the flight example

The flight package provides IntegrationTestClient and IntegrationTestServer as sample implementations for client/server side.

In these implementations, the client sends the content of some json file to the server. However, it is not clear what the format of the json file should be like.

So it is desirable to also provide a sample json file, which makes it easier to run the flight program.

**Reporter**: [Liya Fan](https://issues.apache.org/jira/browse/ARROW-5822) / @liyafan82

_{**Note**: *This issue was originally created as [ARROW-5822](https://issues.apache.org/jira/browse/ARROW-5822). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

首先查看 flight package 中的 IntegrationTestClient 和 IntegrationTestServer 实现，以了解客户端发送哪些 JSON 内容以及服务器期望什么内容。添加一个符合该格式的 JSON 示例文件，并验证 flight example 能够成功使用它。

由索引模型根据 Issue 内容生成。