[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.*}

Start with the flight package's IntegrationTestClient and IntegrationTestServer implementations to see which JSON content the client sends and what the server expects. Add a sample JSON file that matches that format and verify the flight example can use it successfully.

Written by the indexing model from the issue text.