Add cross post information to the api

[jwr1]

Basically, the title. At the moment, cross posts (or related posts) can only be viewed through the web client. It would be great if cross post data were also available through the API.

Presumably, this would be a key in the entry object (e.g., crossPosts) with an array of related posts with the exact same structure as the parent entry data (minus the crossPosts key).