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/schema_change/migration_schema_v27.rs

@@ -0,0 +1,26 @@
+use crate::BeaconChainTypes;
+use std::sync::Arc;
+use store::{metadata::SchemaVersion, Error, HotColdDB};
+
+/// Add `DataColumnCustodyInfo` entry to v27.
+pub fn upgrade_to_v27<T: BeaconChainTypes>(
+    db: Arc<HotColdDB<T::EthSpec, T::HotStore, T::ColdStore>>,
+) -> Result<(), Error> {
+    if db.spec.is_peer_das_scheduled() {
+        db.put_data_column_custody_info(None)?;
+        db.store_schema_version_atomically(SchemaVersion(27), vec![])?;
+    }
+
+    Ok(())
+}
+
+pub fn downgrade_from_v27<T: BeaconChainTypes>(
+    db: Arc<HotColdDB<T::EthSpec, T::HotStore, T::ColdStore>>,
+) -> Result<(), Error> {
+    if db.spec.is_peer_das_scheduled() {
+        return Err(Error::MigrationError(
+            "Cannot downgrade from v27 if peerDAS is scheduled".to_string(),
+        ));
+    }
+    Ok(())
+}