Integrate libp2p mix into lightpush

[jm-clius]

Owner: core research

Feature: Mix

FURPS:

• F1. Relay nodes can mount mixnet protocol, acting as sender, intermediary or exit nodes.

• F2. Nodes can discover other nodes that support mix using available peer discovery mechanisms

• F3. Client nodes can send light push requests over the mixnet before delivery to a service node.

• F4. Client nodes can receive a response to a light push request over the mixnet.

• S1. wakunode2 for intermediary and exit nodes.

• S2. nwaku CLI for sender nodes.

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) <-- Not really applicable, as mix is hardcoded config for testing now. Some instructions, e.g. use mix specific image and enable mix logos-delivery-compose#178

[jm-clius]

@chaitanyaprem this is the mixnet deliverable. Please create subissues (probably one for the spec and one for the implementation) in the research repo as you start on these tasks.

[chair28980]

Assigned Deliverable ownership to @jm-clius and tasks assigned to @chaitanyaprem 🤗