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>

consensus/proto_array/src/error.rs

@@ -1,9 +1,10 @@
-use types::{Checkpoint, Epoch, Hash256};
+use types::{Checkpoint, Epoch, ExecutionBlockHash, Hash256};
 
 #[derive(Clone, PartialEq, Debug)]
 pub enum Error {
     FinalizedNodeUnknown(Hash256),
     JustifiedNodeUnknown(Hash256),
+    NodeUnknown(Hash256),
     InvalidFinalizedRootChange,
     InvalidNodeIndex(usize),
     InvalidParentIndex(usize),
@@ -15,6 +16,7 @@ pub enum Error {
     DeltaOverflow(usize),
     ProposerBoostOverflow(usize),
     IndexOverflow(&'static str),
+    InvalidExecutionDeltaOverflow(usize),
     InvalidDeltaLen {
         deltas: usize,
         indices: usize,
@@ -26,7 +28,21 @@ pub enum Error {
     InvalidBestNode(Box<InvalidBestNodeInfo>),
     InvalidAncestorOfValidPayload {
         ancestor_block_root: Hash256,
-        ancestor_payload_block_hash: Hash256,
+        ancestor_payload_block_hash: ExecutionBlockHash,
+    },
+    ValidExecutionStatusBecameInvalid {
+        block_root: Hash256,
+        payload_block_hash: ExecutionBlockHash,
+    },
+    InvalidJustifiedCheckpointExecutionStatus {
+        justified_root: Hash256,
+    },
+    UnknownLatestValidAncestorHash {
+        block_root: Hash256,
+        latest_valid_ancestor_hash: Option<ExecutionBlockHash>,
+    },
+    IrrelevantDescendant {
+        block_root: Hash256,
     },
 }