Altair validator client and HTTP API (#2404)

## Proposed Changes

* Implement the validator client and HTTP API changes necessary to support Altair


Co-authored-by: realbigsean <seananderson33@gmail.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/http_api/tests/main.rs

@@ -0,0 +1,6 @@
+#![cfg(not(debug_assertions))] // Tests are too slow in debug.
+#![recursion_limit = "256"]
+
+pub mod common;
+pub mod fork_tests;
+pub mod tests;