Create guidance for additional properties

[markweitzel]

In Azure, we have additionalProperties that in swagger is just a free-form JSON object. While the free-form object allow greater expression, it also forces customer off a cliff where they have to get the entire JSON response, peel off the additionalProperties object and deserialize it themselves into their own customer-defined structure and/.or do a bunch of runtime casting/type conversions. Maybe we want to consider a guideline on "additional properties" in general (since data-plane has no guideline for this now) and perhaps we want to adopt the map<string, string> approach as this is really easy to expose/model in all programming languages.