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.
And to make it such that each endpoint resource doesn't need to define the parameter type (like we had to do before, [see this](https://github.com/dickolsson/drupal-relaxed/blob/7ad657810ddb3f2c48bc7b66c9fbb730d84eddc1/src/Plugin/rest/resource/DocResource.php#L23)) we need to make so that we name the parameter `{entity_uuid:file}` instead of `{uuid}` in the route. Because then we can add the parameter type dynamically to the route. See how I do it for the `DocIdConverter` and `DbConverter` [here](https://github.com/dickolsson/drupal-relaxed/blob/8.x-1.x/src/Plugin/rest/resource/ResourceBase.php#L44).