Subscriptions schema needs keys defined.

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

## What are examples of products that are impacted by the issue
IDS, Unified Profile, Campaign

The current draft of https://github.com/adobe/xdm/blob/master/schemas/context/subscription.schema.json was designed directly from legacy implementations. Thus it has definitions that were not intentionally designed for big data or unified profile usage.

- [ ] The current `xdm:subscriber` is an instance of the `person.schema.json`. It should really be a key to `identity.schema.json' so it can be used as a join.
- [ ] Profile needs to have a list of 'subscriptions' that the profile is related to (can be a mix of individual or group subscriptions)
- [ ] All subscription description need to be reviewed and improved where they are not descriptive enough to develop a compatible dataset.
- [ ] Addition examples to cover group and individual subscriptions as well as various types or subscription (usage, time, seats etc)

Start with schemas/context/subscription.schema.json and compare its xdm:subscriber definition with person.schema.json and identity.schema.json. Review how profile subscriptions are represented, then define the affected subscription relationships and examples for individual, group, usage, time, and seat subscriptions. Done means the schema descriptions, keys, profile relationship, and examples cover the requested cases.

Written by the indexing model from the issue text.