2282307 attachment endpoint

Implementation of Attachment endpoint. 

What is not yet implemented/blockers: 
- For get request the value of "Content-Type" header is not correct in the response headers, it should be the mime type of the attachment.
- On put request the file should be attached to the entity as a new value for field_test_file.
We should use the service `file.mime_type.guesser` (`Drupal\Core\File\MimeType\MimeTypeGuesser`) to map MIME type to a valid `Content-Type` header. Because MIME type and Content-Type are not exactly the same thing.