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>

beacon_node/src/cli.rs

@@ -466,8 +466,9 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .takes_value(true)
         )
         .arg(
-            Arg::with_name("payload-builder")
-                .long("payload-builder")
+            Arg::with_name("builder")
+                .long("builder")
+                .alias("payload-builder")
                 .alias("payload-builders")
                 .help("The URL of a service compatible with the MEV-boost API.")
                 .requires("execution-endpoint")

beacon_node/src/config.rs

@@ -288,10 +288,10 @@ pub fn get_config<E: EthSpec>(
             parse_only_one_value(&secret_files, PathBuf::from_str, "--execution-jwt", log)?;
 
         // Parse and set the payload builder, if any.
-        if let Some(endpoints) = cli_args.value_of("payload-builder") {
+        if let Some(endpoint) = cli_args.value_of("builder") {
             let payload_builder =
-                parse_only_one_value(endpoints, SensitiveUrl::parse, "--payload-builder", log)?;
-            el_config.builder_endpoints = vec![payload_builder];
+                parse_only_one_value(endpoint, SensitiveUrl::parse, "--builder", log)?;
+            el_config.builder_url = Some(payload_builder);
         }
 
         // Set config values from parse values.