Mixin to include segmentOwner and FeedName for external segmentdefinition schema

Add a new mixin to include the segmentOwner and FeedName of external segment definitions.
The mixin intendsToExtend the segmentdefinition standard class.

## What are the schemas that are affected by the issue
- None;
## What are examples of products that are impacted by the issue
- 2P Audience Sharing - A new initiative by AEP.

The issue names no files, tests, or entry points to inspect. Start by locating the segmentdefinition standard class and existing mixins, then define the scope for a mixin that includes segmentOwner and FeedName and intendsToExtend that class; done should include the new mixin and validation for the affected schema behavior.

Written by the indexing model from the issue text.