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)

validator_client/signing_method/src/web3signer.rs

@@ -30,6 +30,7 @@ pub enum ForkName {
     Deneb,
     Electra,
     Fulu,
+    Gloas,
 }
 
 #[derive(Debug, PartialEq, Serialize)]
@@ -113,6 +114,11 @@ impl<'a, E: EthSpec, Payload: AbstractExecPayload<E>> Web3SignerObject<'a, E, Pa
                 block: None,
                 block_header: Some(block.block_header()),
             }),
+            BeaconBlock::Gloas(_) => Ok(Web3SignerObject::BeaconBlock {
+                version: ForkName::Gloas,
+                block: None,
+                block_header: Some(block.block_header()),
+            }),
         }
     }