feat(api): Add Image type for built-in cover property

[cvauclair]

Description

Add a dedicated Image GraphQL type with additional dimension information for the built-in cover property.

Implementation

type Image {
  url: String!
  width: Int
  height: Int
  fileType: String
}

type Entity {
  # (Change) cover from `String` to `Image` 
  cover: Image

  # ...other attribute
}