Introduce hooks for Cluster subscribe and unsubscribe

[J-Gras]

I have a use case where I want to know which topics a node is subscribed to. For this, I'd like to have hooks like Cluster::on_subscribe(topic_prefix: string) and Cluster::on_unsubscribe(topic_prefix: string) to maintain a list of currently subscribed topics.

[0xxon]

Out of curiosity - could you perhaps sketch the use-case for this?

[J-Gras]

The gist: I have an external process that needs to understand where messages will flow to do synchronization. It's a little bit weird, because it's somehow contrary to the idea behind a publish-subscribe-architecture 🤷