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_testnet_config/Cargo.toml

@@ -8,6 +8,9 @@ build = "build.rs"
 
 [build-dependencies]
 reqwest = { version = "0.10.4", features = ["blocking"] }
+eth2_config = { path = "../eth2_config"}
+handlebars = "3.3.0"
+serde_json = "1.0.56"
 
 [dev-dependencies]
 tempdir = "0.3.7"
@@ -18,3 +21,4 @@ serde_yaml = "0.8.11"
 types = { path = "../../consensus/types"}
 enr = { version = "0.1.0", features = ["libsecp256k1", "ed25519"] }
 eth2_ssz = "0.1.2"
+eth2_config = { path = "../eth2_config"}