Broadcast a message to multiple channels

Feature Request: Add the ability to broadcast messages to multiple channels simultaneously in Supabase. This feature would allow event payloads to include a list of target channels for broadcasting.



Possible Use Cases:
1. Multi-Group Notifications: Broadcast important announcements or updates to multiple user groups or teams in real-time.

2. Dashboard Updates: Simultaneously update multiple dashboard components with relevant data, ensuring all users see the latest information.

3. Cross-Platform Communication: Efficiently share messages across various communication channels, such as web, mobile, and email, in one go.

4. Event Coordination: Coordinate events or activities by sending event-related messages to participants in multiple event-specific channels.

5. Administrative Alerts: Alert administrators or moderators across different departments or areas about system issues or important events.