refactor: Move proxy components into crate::net

[XAMPPRocky]

This a tranche of #1191, just making the net::io module and moving the components there, doesn't change the API.

[quilkin-bot]

Build Succeeded 🥳

Build Id: 2c963375-2f39-4f2c-8243-2781367e09a0

The following development images have been built, and will exist for the next 30 days:

• us-docker.pkg.dev/quilkin/ci/quilkin:0.10.0-dev-f82ed98

To build this version:

git fetch [email protected]:googleforgames/quilkin.git pull/1207/head:pr_1207 && git checkout pr_1207
cargo build