Allow an page with PageType = API to be embedded as a page part on another page of type API. This will solve the problem of allowing complex types in an API but will also make the sub-entity accessible as its own API.

We don't want a new PageType of APIPart, just make it so any API type page can be added to an existing API type page as a part wherever you can put a field.

The API Part would have link properties that would require you to set the OData Key Fields for the sub part to values derived from the parent (just like a page part does now).