[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.*}

Inizia dalle implementazioni di IntegrationTestClient e IntegrationTestServer del flight package per vedere quale contenuto JSON invia il client e cosa si aspetta il server. Aggiungi un file JSON di esempio che corrisponda a quel formato e verifica che il flight example possa usarlo correttamente.

Scritto dal modello di indicizzazione a partire dal testo della issue.