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/http_api/src/lib.rs

@@ -2138,6 +2138,12 @@ pub fn serve<T: BeaconChainTypes>(
                             query.slot,
                             &query.attestation_data_root,
                         )
+                        .map_err(|e| {
+                            warp_utils::reject::custom_bad_request(format!(
+                                "unable to fetch aggregate: {:?}",
+                                e
+                            ))
+                        })?
                         .map(api_types::GenericResponse::from)
                         .ok_or_else(|| {
                             warp_utils::reject::custom_not_found(