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/cli.rs

@@ -258,4 +258,12 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     immediately.")
                 .takes_value(false),
         )
+        .arg(
+            Arg::with_name("private-tx-proposals")
+                .long("private-tx-proposals")
+                .help("If this flag is set, Lighthouse will query the Beacon Node for only block \
+                    headers during proposals and will sign over headers. Useful for outsourcing \
+                    execution payload construction during proposals.")
+                .takes_value(false),
+        )
 }