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

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

## Implementation
```graphql
type Image {
  url: String!
  width: Int
  height: Int
  fileType: String
}

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

  # ...other attribute
}
```