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
}
```