Support async Listen/Notify API

[fabianfett]

The current Listen/Notify is not usable in a structured concurrency context. We should provide something better:

Goals

1. Provide a new method on connection:

   public func listen(_ channel: String) async throws -> PostgresNotificationSequence

2. The new PostgresNotificationSequence can be backed by an AsyncThrowingStream<String> internally. However we should not expose this, as this gives us greater flexibility down the road.

3. We should create a ListenStateMachine in the PostgresConnection that we update for each Listen channel.

[fabianfett]

Work has started in #264.