thermal: want a mechanism to disable components instead of the main system

As the thermal management subsystem works today, when any sensor reaches its model.should_power_down value, which gives us a ControlResult::PowerDown, with the ultimate result of thermal task calling the power_down() method of its BSP. I think what we want here is a way for a model to specify what should be powered down. For example: if a transceiver module were to to have reported a temperature above the power down threshold in our model for it, the entire switch would power down. This behavior is not acceptable given the failure mode. The behavior we'd want is for the thermal model to optionally specify a device to shut down instead of the whole system.

Start with the thermal management flow from model.should_power_down through ControlResult::PowerDown, the thermal task, and the BSP power_down() method. Trace how a model currently signals a system shutdown, then define completion as allowing it to optionally identify a component to shut down instead of powering down the entire system.