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>

validator_client/src/config.rs

@@ -55,6 +55,7 @@ pub struct Config {
     /// If true, enable functionality that monitors the network for attestations or proposals from
     /// any of the validators managed by this client before starting up.
     pub enable_doppelganger_protection: bool,
+    pub private_tx_proposals: bool,
     /// A list of custom certificates that the validator client will additionally use when
     /// connecting to a beacon node over SSL/TLS.
     pub beacon_nodes_tls_certs: Option<Vec<PathBuf>>,
@@ -91,6 +92,7 @@ impl Default for Config {
             monitoring_api: None,
             enable_doppelganger_protection: false,
             beacon_nodes_tls_certs: None,
+            private_tx_proposals: false,
         }
     }
 }
@@ -306,6 +308,10 @@ impl Config {
             config.enable_doppelganger_protection = true;
         }
 
+        if cli_args.is_present("private-tx-proposals") {
+            config.private_tx_proposals = true;
+        }
+
         Ok(config)
     }
 }