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>

beacon_node/http_api/src/custody.rs

@@ -0,0 +1,53 @@
+use beacon_chain::{BeaconChain, BeaconChainTypes};
+use eth2::lighthouse::CustodyInfo;
+use std::sync::Arc;
+use types::EthSpec;
+use warp_utils::reject::{custom_bad_request, custom_server_error};
+
+pub fn info<T: BeaconChainTypes>(
+    chain: Arc<BeaconChain<T>>,
+) -> Result<CustodyInfo, warp::Rejection> {
+    if !chain.spec.is_fulu_scheduled() {
+        return Err(custom_bad_request("Fulu is not scheduled".to_string()));
+    }
+
+    let opt_data_column_custody_info = chain
+        .store
+        .get_data_column_custody_info()
+        .map_err(|e| custom_server_error(format!("error reading DataColumnCustodyInfo: {e:?}")))?;
+
+    let column_data_availability_boundary = chain
+        .column_data_availability_boundary()
+        .ok_or_else(|| custom_server_error("unreachable: Fulu should be enabled".to_string()))?;
+
+    let earliest_custodied_data_column_slot = opt_data_column_custody_info
+        .and_then(|info| info.earliest_data_column_slot)
+        .unwrap_or_else(|| {
+            // If there's no data column custody info/earliest data column slot, it means *column*
+            // backfill is not running. Block backfill could still be running, so our earliest
+            // available column is either the oldest block slot or the DA boundary, whichever is
+            // more recent.
+            let oldest_block_slot = chain.store.get_anchor_info().oldest_block_slot;
+            column_data_availability_boundary
+                .start_slot(T::EthSpec::slots_per_epoch())
+                .max(oldest_block_slot)
+        });
+    let earliest_custodied_data_column_epoch =
+        earliest_custodied_data_column_slot.epoch(T::EthSpec::slots_per_epoch());
+
+    // Compute the custody columns and the CGC *at the earliest custodied slot*. The node might
+    // have some columns prior to this, but this value is the most up-to-date view of the data the
+    // node is custodying.
+    let custody_context = chain.data_availability_checker.custody_context();
+    let custody_columns = custody_context
+        .custody_columns_for_epoch(Some(earliest_custodied_data_column_epoch), &chain.spec)
+        .to_vec();
+    let custody_group_count = custody_context
+        .custody_group_count_at_epoch(earliest_custodied_data_column_epoch, &chain.spec);
+
+    Ok(CustodyInfo {
+        earliest_custodied_data_column_slot,
+        custody_group_count,
+        custody_columns,
+    })
+}