feat: Custom App Metadata

[stevensJourney]

This is the Dart port of

powersync-ja/powersync-js#783

This allows clients to specify application metadata when calling powerSync.connect. This Record of String values is logged in the PowerSync service logs, which can be used for debugging and analytics. For context: powersync-ja/powersync-service#413

  db.connect(
      connector: currentConnector,
      options: SyncOptions(
          syncImplementation: SyncClientImplementation.rust,
          appMetadata: {'app_version': '1.0.1'}));
}