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.

consensus/context_deserialize/src/lib.rs

@@ -0,0 +1,13 @@
+pub mod impls;
+pub mod milhouse;
+pub mod ssz_impls;
+
+extern crate serde;
+use serde::de::Deserializer;
+
+/// General-purpose deserialization trait that accepts extra context `C`.
+pub trait ContextDeserialize<'de, C>: Sized {
+    fn context_deserialize<D>(deserializer: D, context: C) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>;
+}