Ability for the Task logic to declare itself completed

[meta-paul]

There may be use cases of worker interacting with an intelligent system to complete a task (e.g. win a simple game), where the task itself will have criteria of its completion (it will know when the worker succeeded, and when failed). In these cases, completion of a Task unit will be initiated not by the worker clicking a "Submit" button, but by the Task logic directly adjusting task unit status via provider's API.