Missing field enableSMSMFA on struct events.CognitoEventUserPoolsMigrateUserResponse

**Is your feature request related to a problem? Please describe.**
According to cognito migrate user lambda trigger documentation the event struct could use a field called enableSMSMFA to allow force the migrated user to trigger the SMSMFA step.

That field is not present in the current events package.

**Describe the solution you'd like**
On file `events/cognito.go`:
```
// CognitoEventUserPoolsMigrateUserResponse contains the response portion of a MigrateUser event
type CognitoEventUserPoolsMigrateUserResponse struct {
UserAttributes map[string]string `json:"userAttributes"`
FinalUserStatus string `json:"finalUserStatus"`
MessageAction string `json:"messageAction"`
DesiredDeliveryMediums []string `json:"desiredDeliveryMediums"`
ForceAliasCreation bool `json:"forceAliasCreation"`
EnableSMSMFA bool `json:"enableSMSMFA"`
}
```

**Describe alternatives you've considered**
For now I'm trying to create a local struct that inlude the original plus that field:
```
type UserMigrationEventResponse struct {
events.CognitoEventUserPoolsMigrateUserResponse
EnableSMSMFA bool `json:"enableSMSMFA"`
}

type UserMigrateAuthEvent struct {
events.CognitoEventUserPoolsHeader
events.CognitoEventUserPoolsMigrateUserRequest `json:"request"`
UserMigrationEventResponse `json:"response"`
}
```

**Additional context**
Documentation of the event: https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-migrate-user.html#cognito-user-pools-lambda-trigger-syntax-user-migration