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/execution_layer/src/test_utils/mock_execution_layer.rs

@@ -7,7 +7,7 @@ use sensitive_url::SensitiveUrl;
 use std::sync::Arc;
 use task_executor::TaskExecutor;
 use tempfile::NamedTempFile;
-use types::{Address, ChainSpec, Epoch, EthSpec, Hash256, Uint256};
+use types::{Address, ChainSpec, Epoch, EthSpec, FullPayload, Hash256, Uint256};
 
 pub struct ExecutionLayerRuntime {
     pub runtime: Option<Arc<tokio::runtime::Runtime>>,
@@ -154,7 +154,7 @@ impl<T: EthSpec> MockExecutionLayer<T> {
         let validator_index = 0;
         let payload = self
             .el
-            .get_payload::<T>(
+            .get_payload::<T, FullPayload<T>>(
                 parent_hash,
                 timestamp,
                 prev_randao,
@@ -162,7 +162,8 @@ impl<T: EthSpec> MockExecutionLayer<T> {
                 validator_index,
             )
             .await
-            .unwrap();
+            .unwrap()
+            .execution_payload;
         let block_hash = payload.block_hash;
         assert_eq!(payload.parent_hash, parent_hash);
         assert_eq!(payload.block_number, block_number);