Add a toString method for S3EventNotificationRecord class

Currently while logging S3Event instance , toString just logs hash code .

`S3Event` `toString()` method in turn calls `S3EventNotificationRecord` `toString()` method which is not present . Hence it defaults to plain objects `toString()` method which in turn just logs hash code rendering `toString()` method of `S3Event ` class useless .

https://github.com/aws/aws-lambda-java-libs/blob/4fe08f417a4f5b612bb8d88006da3341eb6d3e51/aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/S3Event.java#L25