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.

從 SQSEvent 類別開始，將其目前的 POJO 建構方式與基於 Lombok 的 KafkaEvent 入口點進行比較。檢視對應的 SNSEvent、S3Event、ScheduleEvent 和 LexEvent 類別，接著為所有事件類型定義一致的 Builder 方法，並驗證 messageAttributes 等巢狀物件可以在不以 null 初始化的情況下建構。

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