Refuse to sign sync committee messages when head is optimistic (#3191)

## Issue Addressed

Resolves #3151 

## Proposed Changes

When fetching duties for sync committee contributions, check the value of `execution_optimistic` of the head block from the BN and refuse to sign any sync committee messages `if execution_optimistic == true`.

## Additional Info
- Is backwards compatible with older BNs
- Finding a way to add test coverage for this would be prudent. Open to suggestions.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1380,10 +1380,41 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     pub fn get_aggregated_sync_committee_contribution(
         &self,
         sync_contribution_data: &SyncContributionData,
-    ) -> Option<SyncCommitteeContribution<T::EthSpec>> {
-        self.naive_sync_aggregation_pool
+    ) -> Result<Option<SyncCommitteeContribution<T::EthSpec>>, Error> {
+        if let Some(contribution) = self
+            .naive_sync_aggregation_pool
             .read()
             .get(sync_contribution_data)
+        {
+            self.filter_optimistic_sync_committee_contribution(contribution)
+                .map(Option::Some)
+        } else {
+            Ok(None)
+        }
+    }
+
+    fn filter_optimistic_sync_committee_contribution(
+        &self,
+        contribution: SyncCommitteeContribution<T::EthSpec>,
+    ) -> Result<SyncCommitteeContribution<T::EthSpec>, Error> {
+        let beacon_block_root = contribution.beacon_block_root;
+        match self
+            .canonical_head
+            .fork_choice_read_lock()
+            .get_block_execution_status(&beacon_block_root)
+        {
+            // The contribution references a block that is not in fork choice, it must be
+            // pre-finalization.
+            None => Err(Error::SyncContributionDataReferencesFinalizedBlock { beacon_block_root }),
+            // The contribution references a fully valid `beacon_block_root`.
+            Some(execution_status) if execution_status.is_valid_or_irrelevant() => Ok(contribution),
+            // The contribution references a block that has not been verified by an EL (i.e. it
+            // is optimistic or invalid). Don't return the block, return an error instead.
+            Some(execution_status) => Err(Error::HeadBlockNotFullyVerified {
+                beacon_block_root,
+                execution_status,
+            }),
+        }
     }
 
     /// Produce an unaggregated `Attestation` that is valid for the given `slot` and `index`.