User-Agent: Add a fallback inspection of "/etc/lsb-release" for Linux OS Info

[arthuraraujo-msft]

          FYI, here's what I implemented for ADU. 

https://github.com/Azure/iot-hub-device-update/blob/10791ac6bd98eaf4e87ea54350d3e9c8a2cec1da/src/platform_layers/linux_platform_layer/src/linux_device_info_exports.cpp#L30

Originally posted by @JeffMill in #182 (comment)