Add SubscriptionFilter type and SubscribeWithFilter method to enable
filtering events by type and actor pattern within namespace subscriptions.
- SubscriptionFilter supports event type filtering (e.g., only "OrderPlaced")
- SubscriptionFilter supports actor ID prefix patterns (e.g., "order-*")
- Filters are combined with AND logic
- NATSEventBus uses NATS subjects for server-side filtering when possible
- Comprehensive test coverage for filtering functionality
Closes#21
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>