Add support for dynamic output parameters

[tkilias]

Background:

• Currently, the mock has the limitation, that it only can run UDF with static output parameters

Acceptance Criteria

• The user has a way to define the default_output_columns() function
• If the user doesn't define output columns, call default_output_columns() before running the run method
• Check if the output of default_output_columns() is valid
• Provide a way that the user can validate the output default_output_columns() regarding its own criteria
• Validate the output of the run method against the output of default_output_columns()