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 create a Param Converter for the `$file_uuid` so that we don't need to do `entity_load()` here. I split the original Entity UUID converter into two specific ones. But you can recover it from here: https://github.com/dickolsson/drupal-relaxed/blob/7ad657810ddb3f2c48bc7b66c9fbb730d84eddc1/src/ParamConverter/EntityUuidConverter.php