Request to add more properties in Principal, and Principal clean-up

## What are the schemas that are affected by the issue
- [ ] https://github.com/adobe/xdm/blob/master/schemas/common/principal.schema.json
- [ ] https://github.com/adobe/xdm/blob/master/extensions/ims/user.schema.json
- [ ] https://github.com/adobe/xdm/blob/master/extensions/ims/organization.schema.json

## What are examples of products that are impacted by the issue
ACP Collaboration services (sharing service, comment service, task service) and products that rely on ACP collaboration services.

## Details -

1. `user` schema composes `principal` but does not extend it. Currently, a user is not a principal.
2. `user` and `organization` schemas both individually redefine similar-sounding properties (some without namespaces) that already exist in `principal` schema. For example, user schema has xdm:identityProvider, id and type. Principal schema has xdm:provider, @id and @type.

I think this is what Leonard suggested as current issues in `principal.schema.json` -

- [ ] @id and @type as used in principal.schema.json is wrong usage of @ in schema. It should have been xdm:id and xdm:type instead. The @id and @type are required properties in the existing principal.schema.json. I can not submit a change to fix this in principal.schema.json as npm run incompatibility-check fails (part of build job in github.com/adobe/xdm). I think the reason for that build step is that schemas can only be extended but not retrospectively changed. In https://github.com/adobe/xdm/blob/master/docs/id.md @id is always supposed to be a URI. In https://github.com/adobe/xdm/blob/master/schemas/common/principal.example.1.json @id is not a URI. It is just a string.
- [ ] `principal` schema is currently non-extensible. So, it is not possible for user or organization schema to extend and override a few properties.

Start with schemas/common/principal.schema.json, extensions/ims/user.schema.json, and extensions/ims/organization.schema.json, then review docs/id.md and principal.example.1.json. Run npm run incompatibility-check to understand the compatibility constraint; done means the principal relationship and property definitions are resolved across the affected schemas without failing that check.

Written by the indexing model from the issue text.