After playing around with it, I'm not sure if the filesystem works, which makes sense given you just have a bucket or repo underneath and the code can be called at unpredictable times. You can, however, add additional javascript files via the package.json and import them, so if you can express your data as JSON or base64, you're good to go. Maybe there is a way to add other types of resources via the package.json but I haven't figured it out yet.

Generally you would interact with Cloud SQL using a regular node MySQL library, since it's just managed MySQL. The API would only be used to automate creating and setting up the CloudSQL instance.