[Deliverable] Specify and implement Kad-DHT discovery protocol and API

[jm-clius]

Owner: AnonComms Discovery

FURPS:

• F1. Libp2p Kad-DHT nodes can advertise their own addresses and capabilities in a signed record
• F2. Libp2p Kad-DHT nodes can discover and validate the signed records of random, advertised peers
• F3. Libp2p Kad-DHT nodes can filter discovered records for specific capabilities
• F4. Libp2p nodes can use a general capability discovery API to discover peers with specific capabilities
• U5. A Kad-DHT discovery protocol and API is published in a specification
• U6. The general capability discovery API is available in Nim and C
• U7. The reference Kad-DHT discovery implementation is completed in Nim over the nim-libp2p Kad-DHT implementation

Checklist:

• Specs: link to specs and/or API definition
• Code: link to GitHub issues/PRs/Epic
• Dogfood: link to dogfooding session/artefact
• Docs: links to README.md or other docs