Disallow attesting to optimistic head (#3140)

## Issue Addressed

NA

## Proposed Changes

Disallow the production of attestations and retrieval of unaggregated attestations when they reference an optimistic head. Add tests to this end.

I also moved `BeaconChain::produce_unaggregated_attestation_for_block` to the `BeaconChainHarness`. It was only being used during tests, so it's nice to stop pretending it's production code. I also needed something that could produce attestations to optimistic blocks in order to simulate scenarios where the justified checkpoint is determined invalid (if no one would attest to an optimistic block, we could never justify it and then flip it to invalid).

## Additional Info

- ~~Blocked on #3126~~

beacon_node/beacon_chain/src/execution_payload.rs

@@ -61,7 +61,7 @@ pub fn notify_new_payload<T: BeaconChainTypes>(
         Ok(status) => match status {
             PayloadStatus::Valid => Ok(PayloadVerificationStatus::Verified),
             PayloadStatus::Syncing | PayloadStatus::Accepted => {
-                Ok(PayloadVerificationStatus::NotVerified)
+                Ok(PayloadVerificationStatus::Optimistic)
             }
             PayloadStatus::Invalid {
                 latest_valid_hash, ..
@@ -193,7 +193,7 @@ pub fn validate_execution_payload_for_gossip<T: BeaconChainTypes>(
 
         let is_merge_transition_complete = match parent_block.execution_status {
             // Optimistically declare that an "unknown" status block has completed the merge.
-            ExecutionStatus::Valid(_) | ExecutionStatus::Unknown(_) => true,
+            ExecutionStatus::Valid(_) | ExecutionStatus::Optimistic(_) => true,
             // It's impossible for an irrelevant block to have completed the merge. It is pre-merge
             // by definition.
             ExecutionStatus::Irrelevant(_) => false,