Ability to provision external services consumed by the deployed application

I'd be cool if external services that need to be provisioned can be as part of the button deploy. Ideally we use `app.json` to specify these. Maybe they are based on the API names that need to be enabled? Should we have some way to select an existing resource? Examples GCS, Cloud SQL, etc.

End goal: An app that needs something like Cloud SQL can be deployed to 100% functionality from the button.