Add selection strategy id to propositionDetails.activity

## 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/proposition-detail.schema.json
https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/decisioning/strategy-detail.schema.json

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

Offer Decisioning / Unified Decisioning

~~Add selection strategy id to activity detail schema~~

- Add strategy schema
- Add selectionStrategy field with strategy schema in proposition-detail

Start by reading decisionevent.schema.json, proposition-detail.schema.json, and strategy-detail.schema.json in extensions/adobe/experience/decisioning. Compare the existing proposition and strategy structures, then verify that the proposition detail includes a selectionStrategy field using the strategy schema and that the affected decision event schema remains consistent.

Written by the indexing model from the issue text.