Addition of userPerspective to application schema

The physical perspective of the user vs the app is needed to properly group or exclude events in session as the majority of the time you will not want to include background or detached events in an active user session.

## What are the schemas that are affected by the issue
https://github.com/adobe/xdm/blob/master/schemas/context/application.schema.json

## What are examples of products that are impacted by the issue
Analytics and any other use-case that needs to understand groupings of events within a session.

Start with schemas/context/application.schema.json, the schema identified in the issue. Review how it represents application events and determine what is needed to capture the user's physical perspective for grouping or excluding background and detached events; done means the affected schema supports the stated Analytics and session use cases.

Written by the indexing model from the issue text.