U4-4618 - public PublishedContentModel & Factory

For review before merge. PublishedContentModel & Factory have been internal in Umbraco for a few months - but do nothing by default. This commit opens them to public, thus letting ppl (should they want to) implement strongly-typed PublishedContentModel factories (no factory is enabled by default). We're NOT providing strongly-typed models. All we do is provide the plumbing so that the content cache can return strongly-typed models.