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/src/lib.rs

@@ -205,6 +205,7 @@ mod test {
         spec.deneb_fork_epoch = Some(Epoch::new(257));
         spec.electra_fork_epoch = None;
         spec.fulu_fork_epoch = None;
+        spec.gloas_fork_epoch = None;
         let result = validator_fork_epochs(&spec);
         assert_eq!(
             result,