Add proposition mixin to AEP Web SDK ExperienceEvent mixin

We need to add the Proposition Interaction mixin, which was provided in https://github.com/adobe/xdm/pull/1139, to the AEP Web SDK ExperienceEvent mixin. Adobe Experience Platform Web SDK will use these fields to notify when personalized content has been rendered or interacted with.

## What are the schemas that are affected by the issue
https://ns.adobe.com/experience/aep-web-sdk-experienceevent
Github URL: https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/aep-web-sdk-experienceevent.schema.json

## What are examples of products that are impacted by the issue
Adobe Experience Platform Web SDK will be populating these fields, which will affect Target and Offer Decisioning.

Open extensions/adobe/experience/aep-web-sdk-experienceevent.schema.json and compare the existing ExperienceEvent mixin structure with the Proposition Interaction mixin described in adobe/xdm pull request 1139. Add the mixin fields so the schema supports notifications when personalized content is rendered or interacted with, then verify the affected schema remains valid.

Written by the indexing model from the issue text.