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/lib.rs

@@ -400,6 +400,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
             .runtime_context(context.service_context("block".into()))
             .graffiti(config.graffiti)
             .graffiti_file(config.graffiti_file.clone())
+            .private_tx_proposals(config.private_tx_proposals)
             .build()?;
 
         let attestation_service = AttestationServiceBuilder::new()