Schema needs updating to support PostgreSQL storage configuration

[guillempuche]

The schema.json file seems outdated as it only allows MongoDB as a storage type, but according to the current documentation, PowerSync now supports PostgreSQL as a storage option.

The schema should be updated to:

1. Allow "postgresql" as a valid storage type
2. Include PostgreSQL-specific configuration options for the storage section
3. Make it consistent with the example configurations in the documentation

Example from docs showing PostgreSQL storage:

storage:
  type: postgresql
  uri: postgresql://user:pass@localhost:5432/db
  sslmode: disable

[stevensJourney]

The schema file in this repository was manually generated and added as a convenience helper for the demos.

Other additions since the initial demo releases, such as MySQL and MongoDB replication are also not yet in this schema file, and would require some additional work to compile all the supported types into a single schema.

Ideally we would generate and publish the schema in the https://github.com/powersync-ja/powersync-service repository. It's not a large project, we'll add this to our internal trackers.