p2p: Replace pubsub peer discovery with ContactInfo based peer discovery

[haardikk21]

Hubble recently updated to no longer do PubSub peer discovery

This is a good thing, because JavaScript LibP2P has pubsub peer discovery available officially - whereas Rust doesn't.

We've so far re-implemented some of the js libp2p stuff for pubsub peer disc, but wasn't fully complete. Ditching it in favor of a ContactInfo based peer discovery is easier and will overall lead to a net-reduction of the amount of work needed on peer disc from us.

[avichalp]

Here is the Hubble reference for handling messages on ContactInfo Topic. It should be a good place to start.

(edit: just noticed that this link was shared in #18 but leaving it here in case someone stumbles on it first)