[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.*}

Beginne mit den Implementierungen von IntegrationTestClient und IntegrationTestServer im flight package, um zu sehen, welche JSON-Inhalte der Client sendet und was der Server erwartet. Füge eine JSON-Beispieldatei hinzu, die diesem Format entspricht, und überprüfe, dass das flight example sie erfolgreich verwenden kann.

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