Optimistic sync spec tests (v1.2.0) (#3564)

## Issue Addressed

Implements new optimistic sync test format from https://github.com/ethereum/consensus-specs/pull/2982.

## Proposed Changes

- Add parsing and runner support for the new test format.
- Extend the mock EL with a set of canned responses keyed by block hash. Although this doubles up on some of the existing functionality I think it's really nice to use compared to the `preloaded_responses` or static responses. I think we could write novel new opt sync tests using these primtives much more easily than the previous ones. Forks are natively supported, and different responses to `forkchoiceUpdated` and `newPayload` are also straight-forward.

## Additional Info

Blocked on merge of the spec PR and release of new test vectors.

testing/ef_tests/src/cases/bls_aggregate_sigs.rs

@@ -7,7 +7,7 @@ use serde_derive::Deserialize;
 #[derive(Debug, Clone, Deserialize)]
 pub struct BlsAggregateSigs {
     pub input: Vec<String>,
-    pub output: String,
+    pub output: Option<String>,
 }
 
 impl_bls_load_case!(BlsAggregateSigs);
@@ -25,14 +25,13 @@ impl Case for BlsAggregateSigs {
             aggregate_signature.add_assign(&sig);
         }
 
-        // Check for YAML null value, indicating invalid input. This is a bit of a hack,
-        // as our mutating `aggregate_signature.add` API doesn't play nicely with aggregating 0
-        // inputs.
-        let output_bytes = if self.output == "~" {
-            AggregateSignature::infinity().serialize().to_vec()
-        } else {
-            hex::decode(&self.output[2..])
-                .map_err(|e| Error::FailedToParseTest(format!("{:?}", e)))?
+        let output_bytes = match self.output.as_deref() {
+            // Check for YAML null value, indicating invalid input. This is a bit of a hack,
+            // as our mutating `aggregate_signature.add` API doesn't play nicely with aggregating 0
+            // inputs.
+            Some("~") | None => AggregateSignature::infinity().serialize().to_vec(),
+            Some(output) => hex::decode(&output[2..])
+                .map_err(|e| Error::FailedToParseTest(format!("{:?}", e)))?,
         };
         let aggregate_signature = Ok(aggregate_signature.serialize().to_vec());