Add support for multiple testnet flags (#1396)

## Issue Addressed

NA

## Proposed Changes

Allows for multiple "hardcoded" testnets.

## Additional Info

This PR is incomplete.

## TODO

- [x] Add flag to CLI, integrate with rest of Lighthouse.


Co-authored-by: Pawan Dhananjay <pawandhananjay@gmail.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>

common/eth2_config/src/lib.rs

@@ -1,4 +1,6 @@
 use serde_derive::{Deserialize, Serialize};
+use std::env;
+use std::path::PathBuf;
 use types::ChainSpec;
 
 /// The core configuration of a Lighthouse beacon node.
@@ -41,6 +43,86 @@ impl Eth2Config {
     }
 }
 
+/// A directory that can be built by downloading files via HTTP.
+///
+/// Used by the `eth2_testnet_config` crate to initialize testnet directories during build and
+/// access them at runtime.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Eth2NetDirectory<'a> {
+    pub name: &'a str,
+    pub unique_id: &'a str,
+    pub commit: &'a str,
+    pub url_template: &'a str,
+    pub genesis_is_known: bool,
+}
+
+impl<'a> Eth2NetDirectory<'a> {
+    /// The directory that should be used to store files downloaded for this net.
+    pub fn dir(&self) -> PathBuf {
+        env::var("CARGO_MANIFEST_DIR")
+            .expect("should know manifest dir")
+            .parse::<PathBuf>()
+            .expect("should parse manifest dir as path")
+            .join(self.unique_id)
+    }
+}
+
+#[macro_export]
+macro_rules! unique_id {
+    ($name: tt, $commit: tt, $genesis_is_known: tt) => {
+        concat!("testnet_", $name, "_", $commit, "_", $genesis_is_known);
+    };
+}
+
+macro_rules! define_net {
+    ($title: ident, $macro_title: tt, $name: tt, $commit: tt, $url_template: tt, $genesis_is_known: tt) => {
+        #[macro_use]
+        pub mod $title {
+            use super::*;
+
+            pub const ETH2_NET_DIR: Eth2NetDirectory = Eth2NetDirectory {
+                name: $name,
+                unique_id: unique_id!($name, $commit, $genesis_is_known),
+                commit: $commit,
+                url_template: $url_template,
+                genesis_is_known: $genesis_is_known,
+            };
+
+            // A wrapper around `std::include_bytes` which includes a file from a specific testnet
+            // directory. Used by upstream crates to import files at compile time.
+            #[macro_export]
+            macro_rules! $macro_title {
+                ($base_dir: tt, $filename: tt) => {
+                    include_bytes!(concat!(
+                        $base_dir,
+                        unique_id!($name, $commit, $genesis_is_known),
+                        "/",
+                        $filename
+                    ))
+                };
+            }
+        }
+    };
+}
+
+define_net!(
+    altona,
+    include_altona_file,
+    "altona",
+    "a94e00c1a03df851f960fcf44a79f2a6b1d29af1",
+    "https://raw.githubusercontent.com/sigp/witti/{{ commit }}/altona/lighthouse/{{ file }}",
+    true
+);
+
+define_net!(
+    medalla,
+    include_medalla_file,
+    "medalla",
+    "b21fef76ddf472c6cea62d5c98b678033a9b195a",
+    "https://raw.githubusercontent.com/sigp/witti/{{ commit }}/medalla/{{ file }}",
+    false
+);
+
 #[cfg(test)]
 mod tests {
     use super::*;