[Proposal] Add a hook to handle stream event without any subscribers

Right now, when a pulling agent get an event for a stream that doesn't have any subscriber, we don't log anything. In some scenarios, it could be useful to send a notification, or log something.

We already have [`IStreamFailureHandler`](https://github.com/dotnet/orleans/blob/main/src/Orleans.Streaming/PersistentStreams/IStreamFailureHandler.cs). It could be added to that interface, or at least serve as a starting point to define another interface.

Start with src/Orleans.Streaming/PersistentStreams/IStreamFailureHandler.cs and trace how pulling agents handle events for streams without subscribers. Clarify whether the existing interface should be extended or a new interface introduced, then define the notification or logging behavior and its completion criteria.

Written by the indexing model from the issue text.