Add experimental complete-blob-backfill flag (#7751)

A different (and complementary) approach for:

- https://github.com/sigp/lighthouse/issues/5391


  This PR adds a flag to set the DA boundary to the Deneb fork. The effect of this change is that Lighthouse will try to backfill _all_ blobs.

Most peers do not have this data, but I'm thinking that combined with `trusted-peers` this could be quite effective.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/beacon_chain/src/chain_config.rs

@@ -86,6 +86,8 @@ pub struct ChainConfig {
     /// If using a weak-subjectivity sync, whether we should download blocks all the way back to
     /// genesis.
     pub genesis_backfill: bool,
+    /// EXPERIMENTAL: backfill blobs and data columns beyond the data availability window.
+    pub complete_blob_backfill: bool,
     /// Whether to send payload attributes every slot, regardless of connected proposers.
     ///
     /// This is useful for block builders and testing.
@@ -144,6 +146,7 @@ impl Default for ChainConfig {
             optimistic_finalized_sync: true,
             shuffling_cache_size: crate::shuffling_cache::DEFAULT_CACHE_SIZE,
             genesis_backfill: false,
+            complete_blob_backfill: false,
             always_prepare_payload: false,
             epochs_per_migration: crate::migrate::DEFAULT_EPOCHS_PER_MIGRATION,
             enable_light_client_server: true,