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/beacon_chain/tests/block_verification.rs

@@ -54,6 +54,7 @@ fn get_harness(validator_count: usize) -> BeaconChainHarness<EphemeralHarnessTyp
         .default_spec()
         .keypairs(KEYPAIRS[0..validator_count].to_vec())
         .fresh_ephemeral_store()
+        .mock_execution_layer()
         .build();
 
     harness.advance_slot();
@@ -839,6 +840,7 @@ fn verify_block_for_gossip_slashing_detection() {
         .keypairs(KEYPAIRS.to_vec())
         .fresh_ephemeral_store()
         .initial_mutator(Box::new(move |builder| builder.slasher(inner_slasher)))
+        .mock_execution_layer()
         .build();
     harness.advance_slot();
 
@@ -918,6 +920,7 @@ fn add_base_block_to_altair_chain() {
         .spec(spec)
         .keypairs(KEYPAIRS[..].to_vec())
         .fresh_ephemeral_store()
+        .mock_execution_layer()
         .build();
 
     // Move out of the genesis slot.
@@ -1036,6 +1039,7 @@ fn add_altair_block_to_base_chain() {
         .spec(spec)
         .keypairs(KEYPAIRS[..].to_vec())
         .fresh_ephemeral_store()
+        .mock_execution_layer()
         .build();
 
     // Move out of the genesis slot.