Sync committee sign bn fallback (sigp#3624)

## Issue Addressed

Closes sigp#3612

## Proposed Changes

- Iterates through BNs until it finds a non-optimistic head.

A slight change in error behavior: 
- Previously: `spawn_contribution_tasks` did not return an error for a non-optimistic block head. It returned `Ok(())` logged a warning.
- Now: `spawn_contribution_tasks` returns an error if it cannot find a non-optimistic block head. The caller of `spawn_contribution_tasks` then logs the error as a critical error.


Co-authored-by: Michael Sproul <[email protected]>

validator_client/src/sync_committee_service.rs

@@ -174,39 +174,40 @@ impl<T: SlotClock + 'static, E: EthSpec> SyncCommitteeService<T, E> {
             return Ok(());
         }
 
-        // Fetch `block_root` and `execution_optimistic` for `SyncCommitteeContribution`.
+        // Fetch `block_root` with non optimistic execution for `SyncCommitteeContribution`.
         let response = self
             .beacon_nodes
-            .first_success(RequireSynced::Yes, OfflineOnFailure::Yes,|beacon_node| async move {
-                beacon_node.get_beacon_blocks_root(BlockId::Head).await
-            })
-            .await
-            .map_err(|e| e.to_string())?
-            .ok_or_else(|| format!("No block root found for slot {}", slot))?;
+            .first_success(
+                RequireSynced::Yes,
+                OfflineOnFailure::Yes,
+                |beacon_node| async move {
+                    match beacon_node.get_beacon_blocks_root(BlockId::Head).await {
+                        Ok(Some(block)) if block.execution_optimistic == Some(false) => {
+                            Ok(block)
+                        }
+                        Ok(Some(_)) => {
+                            Err(format!("To sign sync committee messages for slot {slot} a non-optimistic head block is required"))
+                        }
+                        Ok(None) => Err(format!("No block root found for slot {}", slot)),
+                        Err(e) => Err(e.to_string()),
+                    }
+                },
+            )
+            .await;
 
-        let block_root = response.data.root;
-        if let Some(execution_optimistic) = response.execution_optimistic {
-            if execution_optimistic {
+        let block_root = match response {
+            Ok(block) => block.data.root,
+            Err(errs) => {
                 warn!(
                     log,
-                    "Refusing to sign sync committee messages for optimistic head block";
+                    "Refusing to sign sync committee messages for an optimistic head block or \
+                    a block head with unknown optimistic status";
+                    "errors" => errs.to_string(),
                     "slot" => slot,
                 );
                 return Ok(());
             }
-        } else if let Some(bellatrix_fork_epoch) = self.duties_service.spec.bellatrix_fork_epoch {
-            // If the slot is post Bellatrix, do not sign messages when we cannot verify the
-            // optimistic status of the head block.
-            if slot.epoch(E::slots_per_epoch()) > bellatrix_fork_epoch {
-                warn!(
-                    log,
-                    "Refusing to sign sync committee messages for a head block with an unknown \
-                    optimistic status";
-                    "slot" => slot,
-                );
-                return Ok(());
-            }
-        }
+        };
 
         // Spawn one task to publish all of the sync committee signatures.
         let validator_duties = slot_duties.duties;