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/store/src/lib.rs

@@ -264,6 +264,8 @@ pub enum DBColumn {
     BeaconBlob,
     #[strum(serialize = "bdc")]
     BeaconDataColumn,
+    #[strum(serialize = "bdi")]
+    BeaconDataColumnCustodyInfo,
     /// For full `BeaconState`s in the hot database (finalized or fork-boundary states).
     ///
     /// DEPRECATED.
@@ -424,6 +426,7 @@ impl DBColumn {
             | Self::CustodyContext
             | Self::OptimisticTransitionBlock => 32,
             Self::BeaconBlockRoots
+            | Self::BeaconDataColumnCustodyInfo
             | Self::BeaconBlockRootsChunked
             | Self::BeaconStateRoots
             | Self::BeaconStateRootsChunked