Add a task component kind

[bocchino]

Consider adding a new kind of component task in addition to active, queued, and passive. This would be a component with a task, but no queue. This pattern is useful for blocking on external input, e.g., from a radio. Currently we have to implement this pattern as a passive component with an Os::Task member. It would be better to have a component kind in the model that explicitly declares this pattern. The function that runs on the thread could be pure virtual in the base class and implemented in the derived class.

More generally, we should deprecate the pattern of putting Os::Task members inside components. This pattern makes it impossible to use the FPP model to reason about concurrency. Ideally, every thread in the system should be represented by an active component instance or a task component instance.