Address review comments

- data_availability_checker.rs: use !gloas_enabled() instead of < ForkName::Gloas
  (jimmygchen, dapplion).
- beacon_chain.rs: get_data_columns checks data_availability_checker first,
  then pending_payload_cache (dapplion).
- pending_components.rs: merge_data_columns drops the unused Result return
  (jimmygchen). num_completed_columns uses filter() instead of filter_map (jimmygchen).
- pending_column.rs: TODO marker on the hard-coded Gloas variant in
  try_to_sidecar (jimmygchen).
- pending_payload_cache/mod.rs: gloas_spec test helper collapsed to
  ForkName::Gloas.make_genesis_spec(E::default_spec()) (jimmygchen).
- gossip_methods.rs / sync/manager.rs: replace UnknownBlockHashFromAttestation
  fallback with TODO(gloas) for proper Gloas lookup sync (dapplion).

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1187,9 +1187,9 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         indices: &[ColumnIndex],
     ) -> Result<DataColumnSidecarList<T::EthSpec>, Error> {
         let all_cached_columns_opt = self
-            .pending_payload_cache
+            .data_availability_checker
             .get_data_columns(block_root)
-            .or_else(|| self.data_availability_checker.get_data_columns(block_root))
+            .or_else(|| self.pending_payload_cache.get_data_columns(block_root))
             .or_else(|| self.early_attester_cache.get_data_columns(block_root));
 
         if let Some(mut all_cached_columns) = all_cached_columns_opt {