Add offer content to offer schema

## What are the schemas that are affected by the issue
https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/decisioning/decisionevent.schema.json
https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/decisioning/option-detail.schema.json

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

Changes
Add option content (`xdm:optionContent`) to option schema and this should be an optional field

Open decisionevent.schema.json and option-detail.schema.json, then inspect how existing optional fields are defined in the option schema. Add the optional xdm:optionContent field to option-detail.schema.json and validate the affected schemas to confirm the Offer Decisioning model accepts the new content.

Written by the indexing model from the issue text.