Introduce & standardize xdm:condition

I think XDM should have a "conditional language" to have a standard way to define conditions when a property should be used or not. Conditions could refer to other attributes of the same or another XDM schema.
The language should support basic checks like:
* is / is not present
* equals / not equals
* <, >
* contains (for arrays)

Examples of this are for example in the commerce area where products have prices which only apply to a certain region/customer group/channel/etc.

Example:
```
"xdm:condition": {
"xdm:country": {
"xdm:equals": "DE"
},
"xdm:channel.@type": {
"xdm:equals": "https://ns.adobe.com/xdm/channel-types/web"
}
}
```

No files, tests, or entry points are named. Start by surveying the XDM schemas and documenting a standard for xdm:condition, including presence checks, comparisons, array containment, and references to attributes in the same or another schema; done means the syntax and semantics are specified consistently with the examples.

Written by the indexing model from the issue text.