Alllow simple way to enable peerExchange

[vpavlin]

This is a feature request

Problem

It seems that currently the only straightforward way to enable peerExchange is to set defaultBootstrap: true. This is problematic in case a user runs on a custom pubsubTopic or generally separate network and may not want to use default fleets to bootstrap.

It is possible to enable peerExchange by adding it to libp2p.peerDiscovery (thanks @danisharora099 !)

waku = await createLightNode({
        libp2p: {
          peerDiscovery: [
            wakuPeerExchangeDiscovery(pubsubTopic)
          ]
        }
      });

But it might be useful to either enable it by default, or add something like peerExchange: true to the top level config

Proposed Solutions

(these are or options:) )

1. Enable peerExchange by default
2. Add peerExchange parameter to createLightNode
3. Document the current way (via libp2p.peerDiscovery)

Notes