Enable IPNS over pubsub by default

[lidel]

This depends on #6621 but adds items on top of it.

Ref. https://github.com/ipfs/go-ipfs/blob/master/docs/experimental-features.md#ipns-pubsub

TODO placeholder

• figure out GC for unused topics
  • an idea: IPNS Pubsub Reprovider Duration (GC for unused topics) #8586 IPNS Pubsub Reprovider Duration (GC for unused topics)
• figure out if we need one topic per libp2p-key (baloons nr of subscriptions on public HTTP gateways)
• decide if we tweak the topic name convention to something more compact
• integrate it with Routing.Routers