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.