Add IoTCoreMQTTMessage to aws/aws-lambda-go/events

[lucamarchiori]

Is your feature request related to a problem? Please describe.
Missing type to handle MQTT messages forwarded by IoTCore

Describe the solution you'd like
Add a new type to /aws/aws-lambda-go/events, for example:

type IoTCoreMQTTMessage struct {
	State    IoTCoreMQTTState `json:"state"`
	Topic    string           `json:"topic"`
	ClientId string           `json:"clientd"`
	// ...
}
type IoTCoreMQTTState struct {
	Reported  map[string]interface{} `json:"reported"`
	// ...
}

Describe alternatives you've considered
The user defines the type by himself