Sorting by related entity attributes

[symbioquine]

It should be possible to sort by related entity attributes like this;

await assetLink.entitySource.query(q =>
     q.findRecords('asset--animal')
      .sort('animal_type.name', '-name'));

This is currently supported by Drupal, but not fully by Orbit.js. Two things are needed to make this work;

1. When passing the query to the remote, the query should be augmented with include=animal_type
2. DrupalSyncQueryOperators.js needs to be updated to dereference attributes via relationships from the cache.