Add a CollectionMapper for unspecified entity types
This is useful for representing collection of heterogeneous entities. This mapper would be selected by its regid only, with no extra argument in contrast with other 'jsonschema.collection' mappers that either accepts an "etype" parameter or a relation context. This mapper does not handle submission (i.e. entity creation) since the target entity type is unspecified; so we return a "false" JSON Schema for "creation" role. In tests, we rename prior test case CollectionMapperTC as EntityCollectionMapperTC and add back a CollectionMapperTC explicitly testing the new mapper class.