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.

testing/ef_tests/src/cases/light_client_verify_is_better_update.rs

@@ -3,8 +3,7 @@ use decode::ssz_decode_light_client_update;
 use serde::Deserialize;
 use types::{LightClientUpdate, Slot};
 
-#[derive(Debug, Clone, Deserialize)]
-#[serde(deny_unknown_fields)]
+#[derive(Debug, Clone)]
 pub struct LightClientVerifyIsBetterUpdate<E: EthSpec> {
     light_client_updates: Vec<LightClientUpdate<E>>,
 }