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)

testing/ef_tests/tests/tests.rs

@@ -1,6 +1,7 @@
 #![cfg(feature = "ef_tests")]
 
 use ef_tests::*;
+use std::collections::HashMap;
 use std::path::PathBuf;
 use types::*;
 
@@ -17,6 +18,11 @@ fn config_test<E: EthSpec + TypeName>() {
     let yaml_from_spec = YamlConfig::from_spec::<E>(&spec);
     assert_eq!(yaml_config.apply_to_chain_spec::<E>(&spec), Some(spec));
     assert_eq!(yaml_from_spec, yaml_config);
+    assert_eq!(
+        yaml_config.extra_fields,
+        HashMap::new(),
+        "not all config fields read"
+    );
 }
 
 #[test]