Handle pub/sub better in native pub/sub transports

[SzymonPobiega]

The current handling of pub/sub requires manual configuration in the bridge code. It is not feasible for environments with lots of subscribers. Whenever the topology changes, the bridge needs to be re-deployed.

See the following issue in Router repo:

• NServiceBus 8 support SzymonPobiega/NServiceBus.Router#52 (comment)

[dvdstelt]

Closed in favor of the following issue:

• Auto subscribe to events based on external data #292

It explains some of the history and ideas of why the bridge works the way it does and how to potentially solve it.