Add UnsafeSchedule to PipeScheduler that avoids capturing the ExecutionContext

[davidfowl]

public abstract class PipeScheduler
{
    public virtual void UnsafeSchedule(Action<object> callback, object state);
}

All of the layers above the scheduler will capture and restore the ExecutionContext. We should be able to use UnsafeQueueUserWorkItem here instead. Depends on #27507

cc @pakrym @stephentoub