Move outline-go-tun2socks to a git submodule

[ignoramous]

2. Move `outline-go-tun2socks` in as a `git submodule`.

Would having a monorepo (subfolder/module for tun2socks) also be fine for you? That would be simpler and we could just depend on it using pure gradle (and wouldn't have to init/pull the submodule all the time).
I'm not sure whether Android projects can have non-Android modules though.

We are going to include WireGuard in this project at some point, too: Prefer that it comes in as a git-module. I am mostly basing it off of on how Orchid handles external but critical (security sensitive) cross langauge deps: https://github.com/OrchidTechnologies/orchid/blob/master/.gitmodules

I take, your concern is that gradle process won't be straight-forward? That is, Android Studio build integration would be convoluted more than necessary?

Originally posted by @ignoramous in #212 (comment)

[michaelblyons]

You guys are going to add WireGuard? 🤩 (Edit: it's #52)

[ignoramous]

git submodules plan is defunct because a couple of folks scared me from using it.

Instead, the go-dependencies for RethinkDNS are built from source and published as an Android Archive (firestack.aar) to jitpack.io.