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/src/version.rs

@@ -0,0 +1,28 @@
+use crate::api_types::{EndpointVersion, ForkVersionedResponse};
+use serde::Serialize;
+use types::ForkName;
+
+pub const V1: EndpointVersion = EndpointVersion(1);
+pub const V2: EndpointVersion = EndpointVersion(2);
+
+pub fn fork_versioned_response<T: Serialize>(
+    endpoint_version: EndpointVersion,
+    fork_name: Option<ForkName>,
+    data: T,
+) -> Result<ForkVersionedResponse<T>, warp::reject::Rejection> {
+    let fork_name = if endpoint_version == V1 {
+        None
+    } else if endpoint_version == V2 {
+        fork_name
+    } else {
+        return Err(unsupported_version_rejection(endpoint_version));
+    };
+    Ok(ForkVersionedResponse {
+        version: fork_name,
+        data,
+    })
+}
+
+pub fn unsupported_version_rejection(version: EndpointVersion) -> warp::reject::Rejection {
+    warp_utils::reject::custom_bad_request(format!("Unsupported endpoint version: {}", version))
+}