[eth-pm] Use relay

[fryorcraken]

Problem

web-chat example has been migrated to use filter+light push+store: #87.
As the node leaks IP/content topic link already when using store, it makes sense to use filter and light push.

eth-pm is a good candidate to use relay as store is not used and it's a more privacy focused example (e2ee).

We have relay-reactjs-chat that uses relay but it's an extremely basic app, eth-pm show case a bit more features

Acceptance Criteria

eth-pm uses relay to send and receive messages

Notes

craco needs to be used to enable react-scripts to bundle the .cjs file included in libp2p-gossipsub. See relay-reactjs-chat for inspiration.