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

**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