Improve ergonomics of adding a new network config (#2489)

## Issue Addressed

NA

## Proposed Changes

This PR adds some more fancy macro magic to make it easier to add a new built-in (aka "baked-in") testnet config to the `lighthouse` binary.

Previously, a user needed to modify several files and repeat themselves several times. Now, they only need to add a single definition in the `eth2_config` crate. No repetition 🎉

common/eth2_network_config/build.rs

@@ -1,15 +1,9 @@
 //! Extracts zipped genesis states on first run.
-use eth2_config::{mainnet, prater, pyrmont, Eth2NetArchiveAndDirectory, GENESIS_FILE_NAME};
+use eth2_config::{Eth2NetArchiveAndDirectory, ETH2_NET_DIRS, GENESIS_FILE_NAME};
 use std::fs::File;
 use std::io;
 use zip::ZipArchive;
 
-const ETH2_NET_DIRS: &[Eth2NetArchiveAndDirectory<'static>] = &[
-    mainnet::ETH2_NET_DIR,
-    pyrmont::ETH2_NET_DIR,
-    prater::ETH2_NET_DIR,
-];
-
 fn main() {
     for network in ETH2_NET_DIRS {
         match uncompress_state(network) {