Replace mixnet backend with mixnet addon

[youngjoon-lee]

Summary

• Removed the mixnet backend. Now we have only one network backend: libp2p.
• Added the mixnet add-on (feature) for the libp2p network backend.

Details

I've felt that it's not natural to have the "mixnet network backend" separately from the libp2p network backend. Now I think it's more natural to have only one network backend (libp2p) for now and have mixnet as an addon for the libp2p network backend. There are several reasons:

• If we want to adopt a new network backend such as waku or anything else, do we have to also add another mixnet network backend for that, as below?

  backend/
    libp2p/
    mixnet-libp2p/
    waku/
    mixnet-waku/
  

• The APIs of both libp2p and mixnet network backend are 100% identical. So, I made the libp2p network adapters (for consensus, DA, and mempool) and other parts work for both libp2p and mixnet backend. It was simple to do that, thanks to the conditional compilation in Rust. But, I've felt that code looks unnatural. If both backends are identical, and if both are based on libp2p underneath, why should we have two network backend? I might be able to refactor network adapters to select one of network backends smoothly but I thought it's not really needed for now because we actually have only one networking mechanism (libp2p) now.

There might be several solutions, but I end up thinking about making the mixnet as an add-on for each network backend, so that we can enable/disable mixnet no matter which network backend we choose. Of course, we're going to make the mixnet mandatory in production, but please be remined that we've discussed to make it opt-in for easy development for the time being.
With this structure, we will have only one network backend (libp2p). If we build the nomos-node with only the libp2p feature, the mixnet is disabled.

cargo build --all --no-default-features --features libp2p

If we build it with both libp2p and mixnet feature, the mixnet is enabled for the libp2p network backend.

cargo build --all --no-default-features --features libp2p,mixnet 

[bacv]

I agree that having mixnet as feature in libp2p feels less confusing, nice re-mix.

[danielSanchezQ]

This looks fair. It also makes sense, as we want plain libp2p (for testing mostly) and libp2p+mixnet (production).