Add Gloas boilerplate (#7728)

Adds the required boilerplate code for the Gloas (Glamsterdam) hard fork. This allows PRs testing Gloas-candidate features to test fork transition.

This also includes de-duplication of post-Bellatrix readiness notifiers from #6797 (credit to @dapplion)

beacon_node/beacon_chain/tests/block_verification.rs

@@ -788,6 +788,11 @@ async fn invalid_signature_attester_slashing() {
                     .push(attester_slashing.as_electra().unwrap().clone())
                     .expect("should update attester slashing");
             }
+            BeaconBlockBodyRefMut::Gloas(blk) => {
+                blk.attester_slashings
+                    .push(attester_slashing.as_electra().unwrap().clone())
+                    .expect("should update attester slashing");
+            }
         }
         snapshots[block_index].beacon_block =
             Arc::new(SignedBeaconBlock::from_block(block, signature));
@@ -847,6 +852,10 @@ async fn invalid_signature_attestation() {
                 .attestations
                 .get_mut(0)
                 .map(|att| att.signature = junk_aggregate_signature()),
+            BeaconBlockBodyRefMut::Gloas(blk) => blk
+                .attestations
+                .get_mut(0)
+                .map(|att| att.signature = junk_aggregate_signature()),
         };
 
         if block.body().attestations_len() > 0 {

beacon_node/beacon_chain/tests/store_tests.rs

@@ -184,6 +184,7 @@ async fn light_client_bootstrap_test() {
         LightClientBootstrap::Deneb(lc_bootstrap) => lc_bootstrap.header.beacon.slot,
         LightClientBootstrap::Electra(lc_bootstrap) => lc_bootstrap.header.beacon.slot,
         LightClientBootstrap::Fulu(lc_bootstrap) => lc_bootstrap.header.beacon.slot,
+        LightClientBootstrap::Gloas(lc_bootstrap) => lc_bootstrap.header.beacon.slot,
     };
 
     assert_eq!(