Add a field group for Stitching service

## What are the schemas that are affected by the issue
This field group is meant to be used in the context of an [experience event](https://github.com/adobe/xdm/blob/master/docs/reference/classes/experienceevent.schema.md).

## What are examples of products that are impacted by the issue
(AEP) Adobe Experience Platform and (CJA) Customer Journey Analytics.

Start with the linked Experience Event schema and locate existing field-group definitions for comparable services. Determine the Stitching field group's required properties and placement, then validate the completed schema with the repository's checks.

Written by the indexing model from the issue text.