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>

common/eth2/src/lib.rs

@@ -1334,6 +1334,18 @@ impl BeaconNodeHttpClient {
         self.get(path).await
     }
 
+    /// `GET v1/debug/fork_choice`
+    pub async fn get_debug_fork_choice(&self) -> Result<ForkChoice, Error> {
+        let mut path = self.eth_path(V1)?;
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("debug")
+            .push("fork_choice");
+
+        self.get(path).await
+    }
+
     /// `GET validator/duties/proposer/{epoch}`
     pub async fn get_validator_duties_proposer(
         &self,

common/eth2/src/types.rs

@@ -1197,6 +1197,26 @@ pub struct LivenessResponseData {
     pub is_live: bool,
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+pub struct ForkChoice {
+    pub justified_checkpoint: Checkpoint,
+    pub finalized_checkpoint: Checkpoint,
+    pub fork_choice_nodes: Vec<ForkChoiceNode>,
+}
+
+#[derive(Debug, Serialize, Deserialize, PartialEq, Eq)]
+pub struct ForkChoiceNode {
+    pub slot: Slot,
+    pub block_root: Hash256,
+    pub parent_root: Option<Hash256>,
+    pub justified_epoch: Option<Epoch>,
+    pub finalized_epoch: Option<Epoch>,
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
+    pub weight: u64,
+    pub validity: Option<String>,
+    pub execution_block_hash: Option<Hash256>,
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;