Remove builder redundancy (#3294)

## Issue Addressed

This PR is a subset of the changes in #3134. Unstable will still not function correctly with the new builder spec once this is merged, #3134 should be used on testnets

## Proposed Changes

- Removes redundancy in "builders" (servers implementing the builder spec)
- Renames `payload-builder` flag to `builder`
- Moves from old builder RPC API to new HTTP API, but does not implement the validator registration API (implemented in https://github.com/sigp/lighthouse/pull/3194)



Co-authored-by: sean <seananderson33@gmail.com>
Co-authored-by: realbigsean <sean@sigmaprime.io>

lighthouse/tests/beacon_node.rs

@@ -411,12 +411,13 @@ fn run_payload_builder_flag_test(flag: &str, builders: &str) {
             let config = config.execution_layer.as_ref().unwrap();
             // Only first provided endpoint is parsed as we don't support
             // redundancy.
-            assert_eq!(&config.builder_endpoints, &all_builders[..1]);
+            assert_eq!(config.builder_url, all_builders.get(0).cloned());
         });
 }
 
 #[test]
 fn payload_builder_flags() {
+    run_payload_builder_flag_test("builder", "http://meow.cats");
     run_payload_builder_flag_test("payload-builder", "http://meow.cats");
     run_payload_builder_flag_test("payload-builders", "http://meow.cats,http://woof.dogs");
     run_payload_builder_flag_test("payload-builders", "http://meow.cats,http://woof.dogs");