Retrospective invalidation of exec. payloads for opt. sync (#2837)

## Issue Addressed

NA

## Proposed Changes

Adds the functionality to allow blocks to be validated/invalidated after their import as per the [optimistic sync spec](https://github.com/ethereum/consensus-specs/blob/dev/sync/optimistic.md#how-to-optimistically-import-blocks). This means:

- Updating `ProtoArray` to allow flipping the `execution_status` of ancestors/descendants based on payload validity updates.
- Creating separation between `execution_layer` and the `beacon_chain` by creating a `PayloadStatus` struct.
- Refactoring how the `execution_layer` selects a `PayloadStatus` from the multiple statuses returned from multiple EEs.
- Adding testing framework for optimistic imports.
- Add `ExecutionBlockHash(Hash256)` new-type struct to avoid confusion between *beacon block roots* and *execution payload hashes*.
- Add `merge` to [`FORKS`](c3a793fd73/Makefile (L17)) in the `Makefile` to ensure we test the beacon chain with merge settings.
    - Fix some tests here that were failing due to a missing execution layer.

## TODO

- [ ] Balance tests

Co-authored-by: Mark Mackey <mark@sigmaprime.io>

beacon_node/execution_layer/src/engines.rs

@@ -8,7 +8,7 @@ use lru::LruCache;
 use slog::{crit, debug, info, warn, Logger};
 use std::future::Future;
 use tokio::sync::{Mutex, RwLock};
-use types::{Address, Hash256};
+use types::{Address, ExecutionBlockHash, Hash256};
 
 /// The number of payload IDs that will be stored for each `Engine`.
 ///
@@ -25,9 +25,9 @@ enum EngineState {
 
 #[derive(Copy, Clone, PartialEq, Debug)]
 pub struct ForkChoiceState {
-    pub head_block_hash: Hash256,
-    pub safe_block_hash: Hash256,
-    pub finalized_block_hash: Hash256,
+    pub head_block_hash: ExecutionBlockHash,
+    pub safe_block_hash: ExecutionBlockHash,
+    pub finalized_block_hash: ExecutionBlockHash,
 }
 
 /// Used to enable/disable logging on some tasks.
@@ -48,7 +48,7 @@ impl Logging {
 
 #[derive(Hash, PartialEq, std::cmp::Eq)]
 struct PayloadIdCacheKey {
-    pub head_block_hash: Hash256,
+    pub head_block_hash: ExecutionBlockHash,
     pub timestamp: u64,
     pub random: Hash256,
     pub suggested_fee_recipient: Address,
@@ -75,7 +75,7 @@ impl<T> Engine<T> {
 
     pub async fn get_payload_id(
         &self,
-        head_block_hash: Hash256,
+        head_block_hash: ExecutionBlockHash,
         timestamp: u64,
         random: Hash256,
         suggested_fee_recipient: Address,