The filtering is done by extracting all the related nodes using `extractFieldExample`. The example values are then cached to save some time. (As of this patch, caching is not tested automatically)

Further linking are disabled to avoid cyclic dependencies (input fields only).

TODO: Filtering on nodes linked by mappings and File

Note: If linking is done via array of IDs, only the structure of the first item is extracted. I'll be happy to add union support if someone can show me how to `$in` filter on an array of object.

Related gatsbyjs#3613 gatsbyjs#3190