Implement /lighthouse/custody/info API (#8276)

Closes:

- https://github.com/sigp/lighthouse/issues/8249


  New `/lighthouse/custody` API including:

- [x] Earliest custodied data column slot
- [x] Node CGC
- [x] Custodied columns


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

common/eth2/src/lighthouse.rs

@@ -3,6 +3,7 @@
 mod attestation_performance;
 mod block_packing_efficiency;
 mod block_rewards;
+mod custody;
 pub mod sync_state;
 
 use crate::{
@@ -22,6 +23,7 @@ pub use block_packing_efficiency::{
     BlockPackingEfficiency, BlockPackingEfficiencyQuery, ProposerInfo, UniqueAttestation,
 };
 pub use block_rewards::{AttestationRewards, BlockReward, BlockRewardMeta, BlockRewardsQuery};
+pub use custody::CustodyInfo;
 
 // Define "legacy" implementations of `Option<T>` which use four bytes for encoding the union
 // selector.
@@ -193,6 +195,19 @@ impl BeaconNodeHttpClient {
         self.get(path).await
     }
 
+    /// `GET lighthouse/custody/info`
+    pub async fn get_lighthouse_custody_info(&self) -> Result<CustodyInfo, Error> {
+        let mut path = self.server.full.clone();
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("lighthouse")
+            .push("custody")
+            .push("info");
+
+        self.get(path).await
+    }
+
     /*
      * Note:
      *

common/eth2/src/lighthouse/custody.rs

@@ -0,0 +1,11 @@
+use serde::{Deserialize, Serialize};
+use types::Slot;
+
+#[derive(Debug, PartialEq, Deserialize, Serialize)]
+pub struct CustodyInfo {
+    pub earliest_custodied_data_column_slot: Slot,
+    #[serde(with = "serde_utils::quoted_u64")]
+    pub custody_group_count: u64,
+    #[serde(with = "serde_utils::quoted_u64_vec")]
+    pub custody_columns: Vec<u64>,
+}