How about some Builders in lambda-java-events

The core AWS Java/Kotlin libraries provide builders for just about every object. Builders abound! In the `aws-lambda-java-events` library we get sad old POJOs.

```Kotlin
val messageAttribute = SQSEvent.MessageAttribute()
messageAttribute.stringValue = "So Verbose!"

val sqsMessage = SQSEvent.SQSMessage()
sqsMessage.body = objectMapper.writeValueAsString(DummyMessage(name = "Bob"))
sqsMessage.messageAttributes["So-Much-Typing"] = messageAttribute // <<-- whoops, error, messageAttributes is null
```

I guess I could do this, but its not winning any awards:
```Kotlin
val messageAttribute = SQSEvent.MessageAttribute()
messageAttribute.stringValue = "Still have to do this on two lines"

val sqsMessage = SQSEvent.SQSMessage()
sqsMessage.body = objectMapper.writeValueAsString(DummyMessage(name = "Bob"))
sqsMessage.messageAttributes = mapOf("So-Much-Typing" to messageAttribute)
```

I'm using `SQSEvent` as an example because that's what I was working with today. But same goes for `SNSEvent`, `S3Event`, `ScheduleEvent`, `LexEvent`...

I see that `KafkaEvent` uses Lombok. I'd take that.

Thanks.