The context arguments which are available to main thread callbacks requested using [`run_on_main_thread`](TaskContext::run_on_main_thread).

The Bevy [`Plugin`] which sets up the [`TokioTasksRuntime`] Bevy resource and registers the [`tick_runtime_update`] exclusive system.

The inner fields are boxed to reduce the cost of the every-frame move out of and back into the world in [`tick_runtime_update`].