Gossipsub: add opportunistic grafting in heartbeat

Fixes #5334 (closed).

The spec: https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#opportunistic-grafting.

I followed the Go implementation.

In contrast to the Go/Rust implementation, we explicitly make sure that we do not both prune and graft a peer.