Load block roots from fork choice where possible when serving BlocksByRange requests (#7058)

* Load block roots from fork choice where possible to avoid loading state from disk when serving block by range requests.

* Check if the start slot is newer than finalization (`start_slot >= finalized_slot`), and use fork choice in that case.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -7282,6 +7282,31 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
 
         Ok(None)
     }
+
+    /// Retrieves block roots (in ascending slot order) within some slot range from fork choice.
+    pub fn block_roots_from_fork_choice(&self, start_slot: u64, count: u64) -> Vec<Hash256> {
+        let head_block_root = self.canonical_head.cached_head().head_block_root();
+        let fork_choice_read_lock = self.canonical_head.fork_choice_read_lock();
+        let block_roots_iter = fork_choice_read_lock
+            .proto_array()
+            .iter_block_roots(&head_block_root);
+        let end_slot = start_slot.saturating_add(count);
+        let mut roots = vec![];
+
+        for (root, slot) in block_roots_iter {
+            if slot < end_slot && slot >= start_slot {
+                roots.push(root);
+            }
+            if slot < start_slot {
+                break;
+            }
+        }
+
+        drop(fork_choice_read_lock);
+        // return in ascending slot order
+        roots.reverse();
+        roots
+    }
 }
 
 impl<T: BeaconChainTypes> Drop for BeaconChain<T> {