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.
It doesn't look nice to call `$field_data = $this->serializer->normalize($field, $format, $context);` twice in `ContentEntitySerilizer`. Let's only call it once, and after it's called, put the data into `$data['_attachments']` if it's of the `file` or `image` type.