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/errors.rs

@@ -184,6 +184,9 @@ pub enum BeaconChainError {
     CannotAttestToFinalizedBlock {
         beacon_block_root: Hash256,
     },
+    SyncContributionDataReferencesFinalizedBlock {
+        beacon_block_root: Hash256,
+    },
     RuntimeShutdown,
     TokioJoin(tokio::task::JoinError),
     ProcessInvalidExecutionPayload(JoinError),