Drift demo with riverpod

[simolus3]

This changes the supabase_todolist_drift package to also use Riverpod for state management. This example can then serve as a guide for integrating PowerSync with popular state management solutions (I'll open a PR to our documentation linking towards it).

The idea is to adopt best practices of modern state management in Flutter. This changes the example to:

• Extract the PowerSync database and relevant fields (like the auto-updating SyncState) into Riverpod providers.
• Consume providers from widgets (for simple queries) or through wrapping Notifiers to bundle similar functionality.
• Adopt auto_route for declarative routing.