Caching Image Server plugin

[chyyran]

Implement an image server for image file records. Implements the following API as a plugin (KestrelServerMiddlewareProvider + GraphQL Extension)

scalar Url;
enum Scaling {
  Original,
  Half,
  Quarter,
  Thumbnail,
  Placeholder
}

extend type Query {
  image(fileId: Uuid!) {
      height: Int!
      width: Int!
      mimetype: String!
      url(scaling: Scaling!): Url!
  }
}