Kiln mev boost (#3062)

## Issue Addressed

MEV boost compatibility

## Proposed Changes

See #2987

## Additional Info

This is blocked on the stabilization of a couple specs, [here](https://github.com/ethereum/beacon-APIs/pull/194) and [here](https://github.com/flashbots/mev-boost/pull/20).

Additional TODO's and outstanding questions

- [ ] MEV boost JWT Auth
- [ ] Will `builder_proposeBlindedBlock` return the revealed payload for the BN to propogate
- [ ] Should we remove `private-tx-proposals` flag and communicate BN <> VC with blinded blocks by default once these endpoints enter the beacon-API's repo? This simplifies merge transition logic. 

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

beacon_node/src/cli.rs

@@ -454,6 +454,13 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .requires("merge")
                 .takes_value(true)
         )
+        .arg(
+            Arg::with_name("payload-builders")
+                .long("payload-builders")
+                .help("The URL of a service compatible with the MEV-boost API.")
+                .requires("merge")
+                .takes_value(true)
+        )
 
         /*
          * Database purging and compaction.

beacon_node/src/config.rs

@@ -250,6 +250,14 @@ pub fn get_config<E: EthSpec>(
             el_config.execution_endpoints = client_config.eth1.endpoints.clone();
         }
 
+        if let Some(endpoints) = cli_args.value_of("payload-builders") {
+            el_config.builder_endpoints = endpoints
+                .split(',')
+                .map(SensitiveUrl::parse)
+                .collect::<Result<_, _>>()
+                .map_err(|e| format!("payload-builders contains an invalid URL {:?}", e))?;
+        }
+
         if let Some(secrets) = cli_args.value_of("jwt-secrets") {
             let secret_files: Vec<_> = secrets.split(',').map(PathBuf::from).collect();
             if !secret_files.is_empty() && secret_files.len() != el_config.execution_endpoints.len()