[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.*}

flight package の IntegrationTestClient と IntegrationTestServer の実装から始めて、クライアントが送信する JSON コンテンツとサーバーが期待するものを確認します。その形式に一致する JSON サンプルファイルを追加し、flight example がそれを正常に使用できることを確認します。

索引モデルが issue の本文から書いたものです。