This is a sub-task to WT-17914, only implement the new api_util, the test case for basic config check, and the implementation for fetch_database_size.

For api_util, it's a standalone function which accept an opened connection, a flexible config string which can be extended for future subcommands, and gives a string response to the mongo side, prepared for a new mongo command can be accessed like the way WT-17563 is doing.

For the test case, it should cover the check for config string including valid command, and phrase the output string to simulate a user behaviour.

For fetch_database_size, it should return conn->disaggregated_storage.database_size. And later can be used to check the database size fix actions.