Cognito custom message request type - user attributes type change

**Is your feature request related to a problem? Please describe.**

The `UserAttributes` field on the [`CognitoEventUserPoolsCustomMessageRequest`](https://github.com/aws/aws-lambda-go/blob/master/events/cognito.go#L254-L260) struct has the type `map[string]interface{}` instead of `map[string]string`. The [documentation](https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-custom-message.html#cognito-user-pools-lambda-trigger-syntax-custom-message) for this event states that the values will always be strings. So is the use of `interface{}` intentional?

https://github.com/aws/aws-lambda-go/blob/b3dd246b7d83fd62856549646711e5328075c69a/events/cognito.go#L256

Leaving it as-is requires users of this package to type assert values every time they are extracted from the map which is a little inconvenient.

```go
email := event.Request.UserAttributes["email"].(string)
```

**Describe the solution you'd like**

Change the type of `UserAttributes` to `map[string]string`