Custom API: Not all plugin steps contribute to responses

[Bertie2011]

We are registering our plugins for Custom APIs as pre operation steps, because they can be debugged using the Plugin Registration Tool. Main operation plugins cannot.

Now when trying to write tests we realized that the output parameters set in the pre operation plugin don't show up in the tests. Only output variables set by the main operation will be a available.

It would be nice if all plugin stages could modify the output parameters and influence the final response returned by the framework.