feat: multi-attribute support for expression key builder

[drew-berra]

Problem: The key builder in the expression package only supported exactly two keys, PK + SK. Following the recently released multi-attribute key pattern for GSIs in DDB, a valid query can now support multiple key conditions (DynamoDB currently supports up to 4 partition key + 4 sort key attributes per GSI).

Solution: Update the KeyAnd portion of the expression builder to support multiple keys. Ensure that all but the last key being added are equality, and that the last key is not another KeyAnd expression. Leverage variadic function to maintain backwards compatibility with previous expression building.

Testing: make unit + unit test coverage for new behavior

Issue: None

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

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This PR adds in support for multi-attribute keys in the ddb expression builder. This is in support of a new DDB feature documented here.

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[Madrigal]

Thanks for the contribution. We have some internal work scheduled for this, will wait for leadership decision and come back to you

[raduendv]

@Madrigal @drew-berra seems like a duplicate for #3292 with a slightly different approach

[drew-berra]

@raduendv yes I missed that other PR initially. I took a different approach with my change compared to the PR you linked, but I'm happy with whatever path forward the team takes and looking forward to be able to use the key builder for multi attribute keys