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/tests/tests.rs

@@ -448,6 +448,12 @@ fn fork_choice_ex_ante() {
     ForkChoiceHandler::<MainnetEthSpec>::new("ex_ante").run();
 }
 
+#[test]
+fn optimistic_sync() {
+    OptimisticSyncHandler::<MinimalEthSpec>::default().run();
+    OptimisticSyncHandler::<MainnetEthSpec>::default().run();
+}
+
 #[test]
 fn genesis_initialization() {
     GenesisInitializationHandler::<MinimalEthSpec>::default().run();