New RFC: 39/WAKU2-CAPABILITY-DISCOVERY

[jimstir]

Reference Issue: vacp2p/rfc#520
Author: kaiserd

This issue is part of the Waku ambient discovery roadmap (new roadmap issue to be opened in the research repo; will edit/update this description once the roadmap issue is open)
and tracks the editing of a new RFC specifying a Waku capability discovery protocol.

Background

In its first version, it will simply be libp2p identify
with restrictions on specific fields.
Having a separate Waku specification makes sense because

• the Vac RFC set should comprehensively specify Waku. Capability should not (implicitly) be delegated to an external spec.
• it allows to expand the protocol in the future, which we plan to do.

Also see #521