Distributed pub sub for typed

[johanandren]

The receptionist allows something like distributed pub sub by seing the receptionist registration as a subscription to a topic, it does however not do deduplication, when there are multiple "subscribers" on one node a message for that "topic" will go to all of them.

Could it be worth it to introduce an inbetween-topic-actor that does what distributed pub sub does for a single topic (registering to the receptionist when there are any subscribers, forwarding incoming messages to the subscribers).

[chbatey]

Descoping typed pubsub compared to untyped, what's left:

• Service key describing the topic
• An single topic-actor running on each node (that should take part in the pub-sub) that handles subscriptions to topics and sending to all the other topic-actors that have subscribed to the topic
• actors taking part in pub-sub registers locally to the topic actor
• Depends on de-register as agent should de-register when there are no local subscribers for the topic (in progress) Receptionist: support for actor de-registration  #28123
• Can use the group router internally for sending to get that logic "for free"