Add Battery Status callback

[rickhewes]

Add Battery Status callback

Summary

This PR implements the following feature

• Battery Status callback on DevStatusReq

Adds the ability to change the Battery Status byte when a DevStatusReq is made from the gateway

Validation

uint8_t battery_level_callback()
{
  return random(0, 256);  
}

void setup()
{
   modem.begin(EU868);
   modem.setBatteryLevelCallback(battery_level_callback);
    ...
    ...
}

[fpistm]

LGTM
Thanks for the PR.