Rather than having scattered "if(MONGO_unlikely(serverGlobalParams.configOnly)" throughout the shard registry, we should ideally have a dedicated implementation of the shard registry for configOnly mode use which implements the functions differently than the main shard registry.

Doing this requires substantial refactors since the ShardRegistry currently is not structured in a way to allow this, but changing this would also allow for easier test mocking of the shard registry (since a test implementation could also be used).