Allow experienceevent-stitching field group to be used with additional XDM schema classes

The experienceevent-stitching field group (https://ns.adobe.com/xdm/context/experienceevent-stitching) is currently restricted to use with the experienceevent class. To support broader schema compatibility, this issue proposes allowing the field group to be used with the following additional schema classes:
- https://ns.adobe.com/experience/journeyOrchestration/stepEvents/journeyStepEvent

Start by opening the experienceevent-stitching field group at https://ns.adobe.com/xdm/context/experienceevent-stitching and locating its allowed schema classes in the repository. Add the journeyStepEvent class to that compatibility list, then validate the field group definition and confirm it remains usable with the experienceevent class.

Written by the indexing model from the issue text.