Add support for SearchStax

Problem/Motivation

We are testing migrating from Acquia Search SOLR to SearchStax but during our testing it seems SearchStax has a problem with the "Search API Best Bets" https://www.drupal.org/project/search_api_best_bets → module. After indexing and terms that have a Best Bet configured give an error The result does not contain the essential ID field "ss_search_api_id".

Steps to reproduce
1. Search Stax
2. Add search_api_best_bets and configure it, add a 'best bet' term to a piece of content.
3. Search for that term, see error The result does not contain the essential ID field "ss_search_api_id".

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes