Suport for DynamoDB `attributevalue` `Marshaler` and `Unmarshaler`

[j-d-ha]

Hi, I am a huge fan of your package as it provides a clean way to handle missing and null values in Go, which can be challenging. I am wondering if there is any chance that implementations could be added for the Marshaler and Unmarshaler interfaces from the attributevalue package that's part of the Go AWS SDK. These interfaces define methods that can be used when Unmarshaling response data from DynamoDB and when Marshaling data for use in a dynamo query. If this is something that you have an interest in adding, I'd be happy to work on it.