[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.*}

先從 flight package 的 IntegrationTestClient 和 IntegrationTestServer 實作開始，了解 client 傳送哪些 JSON 內容，以及 server 預期什麼內容。新增一個符合該格式的 JSON 範例檔案，並確認 flight example 能夠成功使用它。

由索引模型根據 Issue 內容生成。