Sync streams

[simolus3]

This adds support for sync streams through the syncStream() method on PowerSyncDatabase.

• I've renamed the sync client from SyncStream to StreamingSyncClient to avoid ambiguity.
• Similar to the Dart and JS implementations, we need to keep track of active subscriptions while we're connected. For Kotlin, a MutableStateFlow that the Rust client subscribes to is a perfect fit for that.
• I've refactored SyncStatusData into an abstract class so that it can have internal fields (such as the one tracking internal Rust stream information).

Otherwise, the implementation is fairly straightforward: Calling db.syncStream(stream, params) returns a pending stream object. Calling subscribe on that will:

• Call powersync_control to update the subscription in the database.
• If this is the first subscription on that stream, update a flow tracking the sync stream subscription. The Rust client will listen on that, do a diff, and reconnect if necessary.
• If we're not connected, update the offline sync status because status information about sync streams is included even while offline (necessary so that clients can selectively unsubscribe from streams even without connecting).