Supporting SMART_BATTERY_INFO

[JonasVautherin]

I got a request to expose the serial_number of SMART_BATTERY_INFO.

From the specs, I read:

Smart Battery information (static/infrequent update). Use for updates from: smart battery to flight stack, flight stack to GCS. Use BATTERY_STATUS for smart battery frequent updates.

Should I try to integrate that to the normal Battery updates of MAVSDK (I guess setting serial_number="" for non-smart batteries), or should it go in a specific SmartBattery type that comes with another subscription?

@hamishwillee @julianoes I wanted to ask for your opinion on that 🙂.

Related to mavlink/MAVSDK-Proto#290

[JonasVautherin]

Closing for now, we'll see if that ever comes back.