Allow starting testnet from JSON state

beacon_node/client/src/config.rs

@@ -56,6 +56,8 @@ pub enum BeaconChainStartMethod {
     Yaml { file: PathBuf },
     /// Create a new beacon chain by loading a SSZ-encoded genesis state from a file.
     Ssz { file: PathBuf },
+    /// Create a new beacon chain by loading a JSON-encoded genesis state from a file.
+    Json { file: PathBuf },
     /// Create a new beacon chain by using a HTTP server (running our REST-API) to load genesis and
     /// finalized states and blocks.
     HttpBootstrap { server: String, port: Option<u16> },

beacon_node/client/src/lib.rs

@@ -147,6 +147,15 @@ where
                 );
                 BeaconChainBuilder::ssz_state(file, spec.clone(), log.clone())?
             }
+            BeaconChainStartMethod::Json { file } => {
+                info!(
+                    log,
+                    "Starting beacon chain";
+                    "file" => format!("{:?}", file),
+                    "method" => "json"
+                );
+                BeaconChainBuilder::json_state(file, spec.clone(), log.clone())?
+            }
             BeaconChainStartMethod::HttpBootstrap { server, port } => {
                 info!(
                     log,