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

[alexandru-tetelea]

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