Add temp field to Stat struct

[swillner]

As per https://github.com/Lora-net/sx1302_hal/blob/master/packet_forwarder/PROTOCOL.md the protocol seems to have been extended to include an (optional) field for temperature in the "stat" object. For monitoring this field can be quite useful and since multiplexers like gwmp-mux interprete these fields, it would be good to add these here, too.

[lthiery]

Sorry I didn't catch this on the way in! Thanks for the PR