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/data_availability_checker.rs

@@ -21,7 +21,7 @@ use tracing::{debug, error, instrument};
 use types::data::{BlobIdentifier, FixedBlobSidecarList, PartialDataColumn};
 use types::{
     BlobSidecar, BlobSidecarList, BlockImportSource, ChainSpec, DataColumnSidecar,
-    DataColumnSidecarList, Epoch, EthSpec, ForkName, Hash256, PartialDataColumnSidecarError,
+    DataColumnSidecarList, Epoch, EthSpec, Hash256, PartialDataColumnSidecarError,
     PartialDataColumnSidecarRef, SignedBeaconBlock, Slot, new_non_zero_usize,
 };
 
@@ -906,7 +906,7 @@ impl<E: EthSpec> AvailableBlock<E> {
         match &block_data {
             AvailableBlockData::NoData => {
                 // For Gloas, DA is checked for the PayloadEnvelope, not for the block.
-                if block.fork_name_unchecked() < ForkName::Gloas {
+                if !block.fork_name_unchecked().gloas_enabled() {
                     if columns_required {
                         return Err(AvailabilityCheckError::MissingCustodyColumns);
                     } else if blobs_required {