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.
Actually, the new `EntityUuidConverter` can almost be as simple as the new `DbConverter` (only use `->loadByUuid()` instead of `->load()`): https://github.com/dickolsson/drupal-relaxed/blob/8.x-1.x/src/ParamConverter/DbConverter.php