Wrap iot-client (#8)

Introduce iot package to wrap the arduino iot-client-go.
This allows to perform cloud operations in a simple and transparent way, like adding a device, exploiting the iot-api rest interface.

* Wrap iot client go

* iot client - implement add certificate

* Refactor iot wrapper as internal package

- Move iot in internal package
- Add significative comments
- Rename auth.go in token.go

Author: Paolo Calao

go.mod

@@ -4,12 +4,14 @@ go 1.15
 
 require (
 	github.com/arduino/arduino-cli v0.0.0-20210607095659-16f41352eac3
+	github.com/arduino/iot-client-go v1.3.3
 	github.com/bcmi-labs/oniudra-cli v0.15.8
 	github.com/eclipse/paho.mqtt.golang v1.3.2
 	github.com/juju/errors v0.0.0-20200330140219-3fe23663418f
 	github.com/sirupsen/logrus v1.8.1
 	github.com/spf13/cobra v1.1.3
 	golang.org/x/net v0.0.0-20210505024714-0287a6fb4125 // indirect
+	golang.org/x/oauth2 v0.0.0-20210628180205-a41e5a781914
 	golang.org/x/sys v0.0.0-20210503173754-0981d6026fa6 // indirect
 	google.golang.org/genproto v0.0.0-20210504143626-3b2ad6ccc450 // indirect
 	google.golang.org/grpc v1.39.0