pubsub: Discover pubsub peers

[dariusc93]

Currently, for nodes to communicate through pubsub topic, they would have to be connected to a peer who is subscribed to the same topic, however when it comes to discovering peers who are apart of the topic, this usually requires the node manual input. We could, instead, add a feature to allow one to enable discovery for pubsub peers of a given topic (or even namespace). Through this, we could allow discovery through DHT, or some rendezvous (either the libp2p protocol or maybe a third party protocol), possibly automate it so we can enable/disable discovery if it falls below or go beyond a specific number of peers for the topic, etc.

Question:

• Do we wish to make it compatible with kubo?
• Should we pass a configuration option to Ipfs::pubsub_subcribe or return a struct that would allow configuring before awaiting for the stream?