Avoid unnecessary database lookups in data column RPC requests (#7897)

This PR is an optimisation to avoid unnecessary database lookups when peer requests data columns that the node doesn't custody (advertised via `cgc`).

e.g. an extreme but realistic example - a full node only store 4 custody columns by default, but it may receive a range request of 32 slots with all 128 columns, and this would result in 4096 database lookups but the node is only able to get 128 (4 * 32) of them.


  - Filter data column RPC requests (`DataColumnsByRoot`, `DataColumnsByRange`) to only lookup columns the node custodies
- Prevents unnecessary database queries that would always fail for non-custody columns

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -7153,6 +7153,17 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             .custody_context()
             .sampling_columns_for_epoch(epoch, &self.spec)
     }
+
+    /// Returns a list of column indices that the node is expected to custody for a given epoch.
+    /// i.e. the node must have validated and persisted the column samples and should be able to
+    /// serve them to peers.
+    ///
+    /// If epoch is `None`, this function computes the custody columns at head.
+    pub fn custody_columns_for_epoch(&self, epoch_opt: Option<Epoch>) -> &[ColumnIndex] {
+        self.data_availability_checker
+            .custody_context()
+            .custody_columns_for_epoch(epoch_opt, &self.spec)
+    }
 }
 
 impl<T: BeaconChainTypes> Drop for BeaconChain<T> {