Data column custody info (#7648)

#7647


  Introduces a new record in the blobs db `DataColumnCustodyInfo`

When `DataColumnCustodyInfo` exists in the db this indicates that a recent cgc change has occurred and/or that a custody backfill sync is currently in progress (custody backfill will be added as a separate PR). When a cgc change has occurred `earliest_available_slot` will be equal to the slot at which the cgc change occured. During custody backfill sync`earliest_available_slot` should be updated incrementally as it progresses.

~~Note that if `advertise_false_custody_group_count` is enabled we do not add a `DataColumnCustodyInfo` record in the db as that would affect the status v2 response.~~
(See comment https://github.com/sigp/lighthouse/pull/7648#discussion_r2212403389)

~~If `DataColumnCustodyInfo` doesn't exist in the db this indicates that we have fulfilled our custody requirements up to the DA window.~~
(It now always exist, and the slot will be set to `None` once backfill is complete)

StatusV2 now uses `DataColumnCustodyInfo` to calculate the `earliest_available_slot` if a `DataColumnCustodyInfo` record exists in the db, if it's `None`, then we return the `oldest_block_slot`.

beacon_node/network/src/status.rs

@@ -29,8 +29,22 @@ pub(crate) fn status_message<T: BeaconChainTypes>(beacon_chain: &BeaconChain<T>)
         finalized_checkpoint.root = Hash256::zero();
     }
 
-    let earliest_available_slot = beacon_chain.store.get_anchor_info().oldest_block_slot;
+    // NOTE: We are making an assumption that `get_data_column_custody_info` wont fail.
+    let earliest_available_data_column_slot = beacon_chain
+        .store
+        .get_data_column_custody_info()
+        .ok()
+        .flatten()
+        .and_then(|info| info.earliest_data_column_slot);
 
+    // If data_column_custody_info.earliest_data_column_slot is `None`,
+    // no recent cgc changes have occurred and no cgc backfill is in progress.
+    let earliest_available_slot =
+        if let Some(earliest_available_data_column_slot) = earliest_available_data_column_slot {
+            earliest_available_data_column_slot
+        } else {
+            beacon_chain.store.get_anchor_info().oldest_block_slot
+        };
     StatusMessage::V2(StatusMessageV2 {
         fork_digest,
         finalized_root: finalized_checkpoint.root,