### Context
As far as we have defined scheme for REST Api:
https://www.mediawiki.org/wiki/Core_Platform_Team/Initiatives/Core_REST_API_in_MediaWiki/Schema
it would be useful to have that represented in the code as Entities.
Say at the moment SearchHandler outputs results that contain the `thumbnail` using SearchResultThumbnail, but on the scheme, it's called FileRepresentation. `File`, according to the scheme should also contain `preferred`, `medium` fields of FileRepresentation type.
Though at the moment SearchResultTumbnails belongs to the `MediaWiki\Search` domain and has nothing common with, REST, in fact, every change in REST/scheme automatically should be reflected in SearchResultTumbnails, especially when adding new fields.
So, the goal is to reduce the mess in naming/modeling and make serialization simpler and more strict.
### Expected Result:
* classes Page, Revision, File, FileRevision, FileRepresentation should be created under MediaWiki\REST\Entities
* Serializers that convert those classes into php array or JSON should be added for each fo file.
* REST handlers should return