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/http_api/src/lib.rs

@@ -3760,7 +3760,6 @@ pub fn serve<T: BeaconChainTypes>(
                                 .to_string(),
                         ));
                     }
-
                     Ok(())
                 })
             },
@@ -3845,6 +3844,12 @@ pub fn serve<T: BeaconChainTypes>(
                             current_slot,
                             &chain.spec,
                         ) {
+                            chain.update_data_column_custody_info(Some(
+                                cgc_change
+                                    .effective_epoch
+                                    .start_slot(T::EthSpec::slots_per_epoch()),
+                            ));
+
                             network_tx.send(NetworkMessage::CustodyCountChanged {
                                 new_custody_group_count: cgc_change.new_custody_group_count,
                                 sampling_count: cgc_change.sampling_count,