Add `then` to the Java DSL

[ricardozanini]

What would you like to be added:

Currently, when creating a workflow as:

 FuncWorkflowBuilder
                .workflow("intelligent-newsletter")
                   .tasks(function("myfunction", Service::doSomething).then("otherTask"))

The .then portion is not currently supported - users must use the Consumer portion with andThen() to compose the task.

Why is this needed:
Improve user experience.