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/handler.rs

@@ -546,6 +546,37 @@ impl<E: EthSpec + TypeName> Handler for ForkChoiceHandler<E> {
     }
 }
 
+#[derive(Derivative)]
+#[derivative(Default(bound = ""))]
+pub struct OptimisticSyncHandler<E>(PhantomData<E>);
+
+impl<E: EthSpec + TypeName> Handler for OptimisticSyncHandler<E> {
+    type Case = cases::ForkChoiceTest<E>;
+
+    fn config_name() -> &'static str {
+        E::name()
+    }
+
+    fn runner_name() -> &'static str {
+        "sync"
+    }
+
+    fn handler_name(&self) -> String {
+        "optimistic".into()
+    }
+
+    fn use_rayon() -> bool {
+        // The opt sync tests use `block_on` which can cause panics with rayon.
+        false
+    }
+
+    fn is_enabled_for_fork(&self, fork_name: ForkName) -> bool {
+        fork_name != ForkName::Base
+            && fork_name != ForkName::Altair
+            && cfg!(not(feature = "fake_crypto"))
+    }
+}
+
 #[derive(Derivative)]
 #[derivative(Default(bound = ""))]
 pub struct GenesisValidityHandler<E>(PhantomData<E>);