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'}));
}

[simolus3]

The API changes and native implementation look good to me. On the web, we would also have to serialize appMetadata so that it reaches the sync worker.

The StartSynchronization extension type in sync_worker_protocol is the relevant message for that, I think serializing to JSON might be the simplest approach which is also what we do for the schema and sync parameters.

In the worker, we'd have to pass this through _ConnectedClient -> referenceSyncTask -> _AddConnection -> _SyncRunner future (what a quest, I should probably look into simplifying that at some point).

[stevensJourney]

Added support for web. Verified the metadata is passed through on web.

[simolus3]

Looks good to me!