Clarify @id for Identity

Issue #419 (PR #420) propose to drop the URI formatted `@id` property in favor of a simple string `id` property.

There is a train of thought along the lines of _All schemes SHALL have the @id property. But it MAY be optional_ (quoting @lrosenthol). I am sympathetic to this thought.

Essentially, the schema looks like

```json
{
"xdm:id": { "type": "string" },
"xdm:namespace": { "$ref": "https://ns.adobe.com/xdm/context/namespace" },
"xdm:xid": { "type": "string" },
"xdm:primary": { "type": "boolean", "default": false }
}
```
(where `xdm:namespace` is a single mandatory property `xdm:code` of type `string`)

The issues are:

* None of the fields is mandatory, so technically an empty object is a "valid" Identity
* `xdm:id` requires `xdm:namespace` to be a proper identity
* `xdm:xid` by itself is unique and is a mapping for `xdm:id` plus `xdm:namespace`
* `@id` is not defined for this schema

So we have to fix the real optionality of the properties and we have to define how to look at the `@id` property.

First of all, having an `@id` property makes sense as a unique identity of something. But having this property optional does not really add value to such an important field as an identity.

Proposal for discussion:

> For the `@id` property we could consider defining an URN namespace, say _adobe-id_ which requires the `xdm:id` and `xdm:namespace.xdm:code` properties in a form like `urn:adobe-id::`. For example `urn:adobe-id:Email:someone@example.com`

(Such a definition, of course, would require the values for `xdm:code` and `xdm:id` to be restricted. See also #432)

*Note*: This issue summary represents the current state of knowledge and discussion. As we evolve the discussion around this issue, the scope of this issue may shift.

## What are the schemas that are affected by the issue

[Identity](https://github.com/adobe/xdm/blob/master/schemas/context/identity.schema.json) I in fact see a number of practical problems.

## What are examples of products that are impacted by the issue

Analytics, Campaign, Ad Cloud, Target

Start with schemas/context/identity.schema.json, then read the linked discussion in issues #419 and #432. Determine the agreed treatment of optional identity fields and @id, including the affected product use cases; done requires a decided scope and corresponding schema definition.

Written by the indexing model from the issue text.