Support DynamoDB's new multi-attribute GSI partition and sort keys

[cluebbehusen]

DynamoDB added a new feature late last year to support multi-attribute GSI partition and sort keys. It would be great if this library could be updated to support this new feature.

https://aws.amazon.com/about-aws/whats-new/2025/11/amazon-dynamodb-multi-attribute-composite-keys-global-secondary-indexes/
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.DesignPattern.MultiAttributeKeys.html