support for domain type extents (or Listable domain types)

[abstratt]

Was looking everywhere in the spec on how to list the extent of a domain type (SELECT * FROM), to no avail, when then saw section 34.6 (Listable Instances), which states it is not supported and considers it as a future idea.

This goes towards supporting the direct manipulation use cases. The benefit over a purposed service is that it provides a uniform way of reaching an objects' extent. Of course, to be practical, it has to be paginated. It could support attribute-based filtering as well, but that would probably belong in a different issue.

[abstratt]

As an example (from a different API with similar principles as RO), see:

http://develop.cloudfier.com/services/api/demo-cloudfier-examples-expenses/entities/

(where each entity a.k.a. 'domain type' has an extent URI, in addition to the "finders" (queries) purposedly defined by the author).