Feature: argument format for the RealTestDataGenerator module should be used to format the Response object

[StijnDeJong]

As of now the format argument in the RealTestDataGenerator is used in the creation of the message for the Prompt object. This should instead be used when formatting the Response object. The user should have multiple options depending on the type of test data generated, such as a list or a dictionary.

[StijnDeJong]

This could potentially also be integrated into the abstract class TestDataGenerator to avoid code duplication, which would be preferred if possible