[Deliverable] Mixnet PoC

[jm-clius]

Summary

Write a short specification describing Waku integration of libp2p mix protocol, including details on interfacing with Waku Discovery methods, spam-protection (RLN), and other Waku-related concerns. This must be followed by a PoC implementation mixing Waku Lightpush and Store requests and responses.

[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.