Segment definition needs to include mapping to equivalent definition from external partners

While developing models for Ad Cloud's search re-marketing features (#477) it was observed that capturing the mapping from our segment definitions to those of an external partner are probably best captured on the segment definition itself, rather than the membership definition.

Since we haven't actually created a segment definition model in XDM yet, we can't make this change now. This issue is being filed to track the need when we do create a segment definition model.

## What are the schemas that are affected by the issue
Segment-oriented schemas in "context"

## What are examples of products that are impacted by the issue
Ad Cloud, Audience Manager

Start by reviewing the segment-oriented schemas in the "context" area and the search re-marketing discussion in #477. The issue is tracking a future segment definition model, so confirm how that model is introduced before deciding where the external-partner mapping belongs. Done means the segment definition captures the equivalent partner definition rather than placing it on the membership definition.

Written by the indexing model from the issue text.