This is to support internal auth within a sharded cluster when shardCollection is moved to the config server.

Internal auth is done through the ShardingConnectionHook for connections from DBConnectionPool (where ScopedDbConnection gets its connection).

However, the ShardingConnectionHook is not added to the DBConnectionPools on config servers (it gets added to globalConnPool and shardConnectionPool on mongod on sharding initialization from a shardIdentity, which doesn't happen on config servers).