Add debug fork choice api (#4003)

## Issue Addressed

Which issue # does this PR address?
https://github.com/sigp/lighthouse/issues/3669

## Proposed Changes

Please list or describe the changes introduced by this PR.
- A new API to fetch fork choice data, as specified [here](https://github.com/ethereum/beacon-APIs/pull/232)
- A new integration test to test the new API

## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.

- `extra_data` field specified in the beacon-API spec is not implemented, please let me know if I should instead.

Co-authored-by: Michael Sproul <micsproul@gmail.com>

consensus/proto_array/src/proto_array_fork_choice.rs

@@ -10,7 +10,10 @@ use crate::{
 use serde_derive::{Deserialize, Serialize};
 use ssz::{Decode, Encode};
 use ssz_derive::{Decode, Encode};
-use std::collections::{BTreeSet, HashMap};
+use std::{
+    collections::{BTreeSet, HashMap},
+    fmt,
+};
 use types::{
     AttestationShufflingId, ChainSpec, Checkpoint, Epoch, EthSpec, ExecutionBlockHash, Hash256,
     Slot,
@@ -125,6 +128,17 @@ impl ExecutionStatus {
     }
 }
 
+impl fmt::Display for ExecutionStatus {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ExecutionStatus::Valid(_) => write!(f, "valid"),
+            ExecutionStatus::Invalid(_) => write!(f, "invalid"),
+            ExecutionStatus::Optimistic(_) => write!(f, "optimistic"),
+            ExecutionStatus::Irrelevant(_) => write!(f, "irrelevant"),
+        }
+    }
+}
+
 /// A block that is to be applied to the fork choice.
 ///
 /// A simplified version of `types::BeaconBlock`.