New RFC: 40/WAKU2-CAPABILITY-PUSH

[jimstir]

Reference issue: vacp2p/rfc#521
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 push protocol.
(also see #520)

Background

In its first version, it will simply be libp2p identify/push
with restrictions on specific fields.

Reasons for having a separate Waku spec instead of simply using libp2p identify/push may be found in #520.

Reasons for having a separte RFC for capability-push and not make it part of #520, even though these protocols are very similar with respect to the transmitted messages:

• these protocols are semantically different
• these protocols might develop in different directions
• imo, each RFC should only specify a single protocol ID.

(Sill, I could merge these two into one, if desired.)