Ensure custody backfill sync couples all responses before importing (#8339)

Custody backfill sync has a bug when we request columns from more than one peer per batch. The fix here ensures we wait for all requests to be completed before performing verification and importing the responses.

I've also added an endpoint `lighthouse/custody/backfill` that resets a nodes earliest available data column to the current epoch so that custody backfill can be triggered. This endpoint is needed to rescue any nodes that may have missing columns due to the custody backfill sync bug without requiring a full re-sync.


  


Co-Authored-By: Eitan Seri- Levi <eserilev@gmail.com>

Co-Authored-By: Eitan Seri-Levi <eserilev@ucsc.edu>

Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

beacon_node/http_api/tests/fork_tests.rs

@@ -1,4 +1,5 @@
 //! Tests for API behaviour across fork boundaries.
+use beacon_chain::custody_context::NodeCustodyType;
 use beacon_chain::{
     StateSkipConfig,
     test_utils::{DEFAULT_ETH1_BLOCK_HASH, HARNESS_GENESIS_TIME, RelativeSyncCommittee},
@@ -426,6 +427,7 @@ async fn bls_to_execution_changes_update_all_around_capella_fork() {
         None,
         Default::default(),
         true,
+        NodeCustodyType::Fullnode,
     )
     .await;
     let harness = &tester.harness;