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/http_api/src/lib.rs

@@ -2324,6 +2324,12 @@ pub fn serve<T: BeaconChainTypes>(
                 blocking_json_task(move || {
                     chain
                         .get_aggregated_sync_committee_contribution(&sync_committee_data)
+                        .map_err(|e| {
+                            warp_utils::reject::custom_bad_request(format!(
+                                "unable to fetch sync contribution: {:?}",
+                                e
+                            ))
+                        })?
                         .map(api_types::GenericResponse::from)
                         .ok_or_else(|| {
                             warp_utils::reject::custom_not_found(