[Deliverable] Implement basic discovery for mix

[chair28980]

Owner: Core Research

Feature: Mix

FURPS:

F5. Nodes can discover other nodes that support mix using available peer discovery mechanisms.

S4. Browser based apps built using js-waku support discovering mix nodes using available peer discovery mechanisms.

Checklist:

• Specs: link to specs
• Code: link to GitHub issues/PRs/Epic
• Dogfood: link to dogfooding session/artefact
• Docs: links to README.md or docs.waku.org (TBD)