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/beacon_chain/src/beacon_chain.rs

@@ -6807,6 +6807,15 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             .map(|duration| (next_digest_epoch, duration))
     }
 
+    /// Update data column custody info with the slot at which cgc was changed.
+    pub fn update_data_column_custody_info(&self, slot: Option<Slot>) {
+        self.store
+            .put_data_column_custody_info(slot)
+            .unwrap_or_else(
+                |e| tracing::error!(error = ?e, "Failed to update data column custody info"),
+            );
+    }
+
     /// This method serves to get a sense of the current chain health. It is used in block proposal
     /// to determine whether we should outsource payload production duties.
     ///