Sync streams

[simolus3]

This depends on powersync-ja/powersync-sqlite-core#112 and powersync-ja/powersync-service#313.

This adds support for sync streams to the Dart SDK by implementing the new methods discussed in the "2025-03 Sync streams" design doc. Namely:

• db.syncStream(name, [params]) returns a SyncStream instance, which can be used to subscribe() to that stream.
• calling subscribe() on a SyncStream returns a SyncStreamSubscription, giving you access to waitForFirstSync() and unsubscribe() on that subscription.
• the SyncStatus interface includes a list of SyncSubscriptionDefinitions, describing all streams that the client is subscribed to (either due to an explicit subscriptions or because the stream has auto_subscribe: true). That interface also reports per-stream download progress.

Additional implementation changes support this:

1. We want a hasSynced and lastSyncedAt for individual streams. The core extension gained a powersync_offline_sync_status() function to help with this, meaning that SDKs no longer have to query the underlying tables directly.
2. In the app itself, active subscriptions (that currently have a SyncStreamSubscription instance attached to them) can change often and quickly. The TTL feature ensures we don't have to recreate a streaming sync socket every time they change. This is mostly implemented in the sync service, we just send a subscriptions_updated event and the Rust client tells us whether to restart the iteration or not.
3. I have moved some connection and subscription management logic into the ConnectionManager class to not complicate the mixin even further.

I have adopted streams in the todolist demo, with the following sync rules:

streams:
  lists:
    query: SELECT * FROM lists
    auto_subscribe: true
  todos:
    query: SELECT * FROM todos WHERE list_id = subscription.parameter('list')

The idea is that lists are shown by default, and entries in a list are loaded and cached when the page is opened for the first time. This works well, but I really don't like that the API currently requires 50 lines of user code for that. We should probably look into a powersync_flutter package with utilities to simplify this eventually.