De-duplicate Ambient LED management code

[fifieldt]

We currently have 3 different places where we have the logic for modifying LED state of the various types of Ambient LEDs,
ExternalNotificationModule::SetExternalState
AmbientLightingThread::setLighting
AmbientLightingThread::setLightingOff

Instead, we should have a single place where this happens.

Use the notification system we have in the firmware for IPC, so that Ambient lighting offers a subscription service for LED changes and the External Notifications uses it (if available).

You can look at how it's done for notifying others about GPS position changes :-)

[Blake-Latchford]

You can look at how it's done for notifying others about GPS position changes :-)

firmware/src/gps/GPS.cpp

Line 975 in e866734

newStatus.notifyObservers(&status);

Is this what you're referring to? If I understand the proposal correctly:

1. AmbientLightingThread should own something like an Observable<LightingConfig> lightChanged.
2. AmbientLightingThread::setLighting is invoked by an observer.
3. LightingConfig probably looks a lot like meshtastic_ModuleConfig_AmbientLightingConfig, but also needs to identify which light is being configured. Though maybe you're expecting that the observer is just watching for ambient lighting.
4. AmbientLightingThread::setLightingOff calls observable.notifyObservers(&offStruct)