add IoT 1-Click event

[shirakiya]

Add event struct for IoT 1-Click ( https://docs.aws.amazon.com/iot-1-click/latest/developerguide/1click-programming.html ).
I assume that we can type the event for lambda handler using this event struct.

import (
  	"github.com/aws/aws-lambda-go/lambda"
	"github.com/aws/aws-lambda-go/events"
)

func handleRequest(ctx context.Context, event events.IoTOneClickEvent) {
	...
}

func main() {
	lambda.Start(handleRequest)
}

I used the sample JSON on the below page to specify the JSON structure.
https://docs.aws.amazon.com/iot-1-click/latest/developerguide/1click-programming.html#1click-events

[bmoffatt]

lgtm, thanks!