In order to enable to optimize the database infrastructure usage by moving `wbc_entity_usage` tables of all wikis to the extension database cluster (x1), Wikibase software should be able to work with multiple database configurations ("get data from multiple database servers")
Notes:
* Mediawiki's virtual database domains is a functionality that provides a helpful way of abstracting database configuration, that is likely going to be of use in this task.
* Relatively similar work has been done by WMDE a few years ago when term-related database tables of wikidata have been moved to another database cluster. See work in T351802 / [I328c1149466761](https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/1105383) for an idea.
* Before the actual move of `wbc_entity_usage` happens, the configuration should point to the current "main" database configuration, to keep the software work as before.
* database queries that read data from `wbc_entity_usage` table and some other Mediawiki table (join) need to be adjusted as those queries would no longer work