Need to mechanism to define schema-level relationships

Need to be able to define schema-level relationships, defined in such a way that customers can manage, modify and extend these relationships based on details of their business.

PR at #154

## What are the schemas that are affected by the issue

N/A - this is a new function

## What are examples of products that are impacted by the issue

Profile building and segmentation, data warehouse

Start by reviewing PR #154 and the issue's schema-level relationship requirements. Define what it means for customers to manage, modify, and extend these relationships, and verify that the result supports the cited profile-building, segmentation, and data-warehouse use cases.

Written by the indexing model from the issue text.