Add claims-extraction because it verifies the extension secret

preprocess.go

        // extension secret is enforced by claims extraction
        cl, err := helper.claims(token)
        if err != nil {
                log.Print("Malformed claims meta data")
                return newResponse("Wrong authorization header", http.StatusUnauthorized),
errors.New("claims")
        }

        log.Printf("Claims (ch/role): %s / %s", cl.ChannelID, cl.Role)

        return events.APIGatewayProxyResponse{}, nil
}