Add PostgreSQL LISTEN/NOTIFY notification listening

Add PostgreSQL LISTEN/NOTIFY support to Objo so apps can receive asynchronous PostgreSQL notifications.

Users who build on PostgreSQL's LISTEN/NOTIFY need a way to subscribe to notification channels and receive notifications while the app runs. Sending notifications already works through the existing database API (NOTIFY or pg_notify() via ExecuteSQL/SelectSQL), but receiving them is not possible today.

A new PostgreSQLNotificationListener class would open its own dedicated PostgreSQL connection, subscribe to one or more channels, and raise an event on the VM thread for each notification received, with the channel name, payload, and sender process ID. Connection details would follow the existing PostgreSQLDatabase conventions (connection string or host/port/database/user/password, Timeout), and connection failures would be reported through an error event with DatabaseException details.