ContextDeserialize and Beacon API Improvements (#7372)

* #7286
* BeaconAPI is not returning a versioned response when it should for some V1 endpoints
* these [strange functions with vX in the name that still accept `endpoint_version` arguments](https://github.com/sigp/lighthouse/blob/stable/beacon_node/http_api/src/produce_block.rs#L192)

This refactor is a prerequisite to get the fulu EF tests running.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -6115,7 +6115,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                         payload_attributes: payload_attributes.into(),
                     },
                     metadata: Default::default(),
-                    version: Some(self.spec.fork_name_at_slot::<T::EthSpec>(prepare_slot)),
+                    version: self.spec.fork_name_at_slot::<T::EthSpec>(prepare_slot),
                 }));
             }
         }