Support facets based on related fields

Thanks so much for this module! It would be great if it would also support facets that are based on related fields. Here's an example:

- Say I'm indexing the taxonomy term name instead of the whole taxonomy term entity, and built a facet based on that field.
- My facet query string will use the term name, so it will be something like f[0]=color:blue.
- Currently, this module will always generate facet query strings using the referenced entity's id, since it passes in $entity->id() as raw value when it generates a new Result object in EntityReferenceFacetFormatterBase::viewElements. As a result, the query string will be something like f[0]=color:123, and the links won't point to a working faceted search page.

This patch does two things:

- The patch adds a setting to use the referenced entity's label instead of its id as the raw value when generating a new Result object in EntityReferenceFacetFormatterBase::viewElements. I'm actually not sure if we need to pass in a $display_value there either, since it seems to work without passing one in (see what Facets module does in FacetsUrlGenerator::getUrl - line 88).
- The patch also broadens the selection of facet candidates in the field configuration (EntityReferenceFacetFormatterBase::settingsForm), so it finds facets based on related fields.