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.

beacon_node/execution_layer/src/engine_api/json_structures.rs

@@ -1,5 +1,6 @@
 use super::*;
 use serde::{Deserialize, Serialize};
+use strum::EnumString;
 use types::{EthSpec, ExecutionBlockHash, FixedVector, Transaction, Unsigned, VariableList};
 
 #[derive(Debug, PartialEq, Serialize, Deserialize)]
@@ -311,8 +312,9 @@ impl From<JsonForkChoiceStateV1> for ForkChoiceState {
     }
 }
 
-#[derive(Debug, PartialEq, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, EnumString)]
 #[serde(rename_all = "SCREAMING_SNAKE_CASE")]
+#[strum(serialize_all = "SCREAMING_SNAKE_CASE")]
 pub enum JsonPayloadStatusV1Status {
     Valid,
     Invalid,