VC: accept unknown fields in chain spec (#2277)

## Issue Addressed

Closes #2274

## Proposed Changes

* Modify the `YamlConfig` to collect unknown fields into an `extra_fields` map, instead of failing hard.
* Log a debug message if there are extra fields returned to the VC from one of its BNs.

This restores Lighthouse's compatibility with Teku beacon nodes (and therefore Infura)

beacon_node/eth2_libp2p/src/behaviour/mod.rs

@@ -36,7 +36,7 @@ use ssz::Encode;
 use std::collections::HashSet;
 use std::fs::File;
 use std::io::Write;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::{
     collections::VecDeque,
     marker::PhantomData,
@@ -1336,7 +1336,7 @@ impl<TSpec: EthSpec> std::convert::From<Response<TSpec>> for RPCCodedResponse<TS
 }
 
 /// Persist metadata to disk
-pub fn save_metadata_to_disk<E: EthSpec>(dir: &PathBuf, metadata: MetaData<E>, log: &slog::Logger) {
+pub fn save_metadata_to_disk<E: EthSpec>(dir: &Path, metadata: MetaData<E>, log: &slog::Logger) {
     let _ = std::fs::create_dir_all(&dir);
     match File::create(dir.join(METADATA_FILENAME))
         .and_then(|mut f| f.write_all(&metadata.as_ssz_bytes()))

beacon_node/eth2_libp2p/src/service.rs

@@ -474,7 +474,7 @@ fn strip_peer_id(addr: &mut Multiaddr) {
 
 /// Load metadata from persisted file. Return default metadata if loading fails.
 fn load_or_build_metadata<E: EthSpec>(
-    network_dir: &std::path::PathBuf,
+    network_dir: &std::path::Path,
     log: &slog::Logger,
 ) -> MetaData<E> {
     // Default metadata