[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.*}

Comienza con las implementaciones de IntegrationTestClient e IntegrationTestServer del flight package para ver qué contenido JSON envía el cliente y qué espera el servidor. Añade un archivo JSON de ejemplo que coincida con ese formato y verifica que el flight example pueda usarlo correctamente.

Escrito por el modelo de indexación a partir del texto del issue.