Support for Multi Attribute Composite Key for GSIs

[keremeyuboglu]

Related AWS announcement:

https://aws.amazon.com/about-aws/whats-new/2025/11/amazon-dynamodb-multi-attribute-composite-keys-global-secondary-indexes/

Here is the example doc page:

https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.DesignPattern.MultiAttributeKeys.html

I wanted to create this issue to garner potential support as this looks like a great improvement on Dynamo itself. I don't know how it would be implemented in the current state of the code without breaking changes though.