Allow observation of OTA events

[rjwats]

Observing OTA events could be useful for suspending services or preparing the device for the firmware update.

The simpliest way to provide that feature may be to expose a factory function to allow the creation of the OTA update instance to be customized (assuming the framework doesn't need to perform event driven actions itself). With the the callbacks being available:

The most extensible option is to provide a callback approach similar to the SettingsService class. I think this is the better approach - possibly with event type similar to the Arduino ESP32 connectivity event callbacks:

otaSettingsService.onEvent(std::bind(&MyService::onUpdateStart, this), OTAEvent::START);