feat: allow configurable index for ddb relationships

[dpilch]

Description of changes

CDK / CloudFormation Parameters Changed

Issue #, if available

Description of how you validated changes

Checklist

• [ ] PR description included
• [ ] yarn test passes
• [ ] Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)
• [ ] New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• [ ] Any CDK or CloudFormation parameter changes are called out explicitly

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