Event Type for CognitoEventUserPoolsCustomEmailSender

**Is your feature request related to a problem? Please describe.**
The events types include types for all cognito event triggers except for the custom sender triggers. It seems there should be types for the custom email sender event and custom sms sender event. This is so that the functionality described [here](https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-custom-email-sender.html) can be implemented in a golang lambda.

**Describe the solution you'd like**
I think the solution is to have an event type for CognitoEventUserPoolsCustomEmailSender

**Describe alternatives you've considered**
Trying to use the existing type of CognitoEventUserPoolsCustomMessage since it most closely resembles this function. Naturally I have not tested this extensively to see what data is missing/would cause an error.

My current handler signature looks like
```
func handler(event events.CognitoEventUserPoolsCustomMessage) (events.CognitoEventUserPoolsCustomMessage, error) {
```