Add annotation to specify whether additional properties may be returned for an entity

In OData V3 only opentypes were allowed to return properties not advertised in $metadata.

In OData V4, for increased resiliency, we said that clients should be prepared to handle additional properties on any resource, and the introduction of PATCH as the preferred method for updating allows that to happen without loss.

However, we lost the ability for a service to say that they will never return additional properties (which is probably the default case). Since clients are required to support additional properties we didn't think this was important, but as we look to a json-schema format it becomes interesting for telling clients (i.e., for validation) whether or not additional properties are allowed.