[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.*}

Commencez par les implémentations de IntegrationTestClient et IntegrationTestServer du flight package afin de voir quel contenu JSON le client envoie et ce que le serveur attend. Ajoutez un fichier JSON d’exemple correspondant à ce format et vérifiez que le flight example peut l’utiliser avec succès.

Rédigé par le modèle d'indexation à partir du texte de l'issue.