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/Cargo.toml

@@ -35,3 +35,4 @@ fs2 = "0.4.3"
 beacon_chain = { path = "../../beacon_node/beacon_chain" }
 store = { path = "../../beacon_node/store" }
 fork_choice = { path = "../../consensus/fork_choice" }
+execution_layer = { path = "../../beacon_node/execution_layer" }