add sensor api to determine battery charge

[jconley]

on garmin connect web, if you click the watch icon at the top right, it displays various sensors connected to your garmin device(s) and their associated battery health. would be very useful for checking if your sensors (heart-rate monitor, power meter, wireless shifting, etc) need charging or new batteries.

api url: https://connect.garmin.com/device-service/sensors
response format: json