Lack of Cognito SMS Lambda Trigger

I couldn't seem to find a class available for the SMS Lambda Trigger, see https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-custom-sms-sender.html

This seems relatively easy to add, there are 2 extra fields here:
- `code`
- `type` (this is static for every event sent to the SMS Lambda trigger)

It doesn't require a response but a user of this can just return the empty event or no event, just to keep in line with other lambda events and just in case there is an update in the future, it will easier to move towards this new update.