lorawan: Add userspace-defined battery level callback API

Provide the LoRaWAN stack with an optional callback to be
called whenever the battery level needs to be read.

In the case of callback not being provided, the LoRaWAN
stack will report 255 (battery level unavailable) as per
the LoRaWAN spec.

Signed-off-by: Kuba Sanak <contact@kuba.fyi>