As a Developer I want to create Segments using extended index fields

• Type: Story
• Status: Closed
• Priority: Minor
• Resolution: Discarded
• Affects Version/s: None
• Fix Version/s: None
• Component/s: Segments Editor
• Labels: None
• Epic/Theme:
  • Segmentation
• Epic Link: Developer experience for extending segmentation

Developers can extend the set of indexed fields of an entity. For instance, they can add new fields to the User index, as part of an application. 

To allow Marketers to filter by these extended fields, the ModelEntity should be extensible. This use case is similar to custom fields (see "customField" reserved ComplexEntity field) but requires a separate reserved field (e.g. "extendedField")