Remove builder redundancy (#3294)

## Issue Addressed

This PR is a subset of the changes in #3134. Unstable will still not function correctly with the new builder spec once this is merged, #3134 should be used on testnets

## Proposed Changes

- Removes redundancy in "builders" (servers implementing the builder spec)
- Renames `payload-builder` flag to `builder`
- Moves from old builder RPC API to new HTTP API, but does not implement the validator registration API (implemented in https://github.com/sigp/lighthouse/pull/3194)



Co-authored-by: sean <seananderson33@gmail.com>
Co-authored-by: realbigsean <sean@sigmaprime.io>

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -334,7 +334,7 @@ pub struct BeaconChain<T: BeaconChainTypes> {
     /// Provides information from the Ethereum 1 (PoW) chain.
     pub eth1_chain: Option<Eth1Chain<T::Eth1Chain, T::EthSpec>>,
     /// Interfaces with the execution client.
-    pub execution_layer: Option<ExecutionLayer>,
+    pub execution_layer: Option<ExecutionLayer<T::EthSpec>>,
     /// Stores a "snapshot" of the chain at the time the head-of-the-chain block was received.
     pub(crate) canonical_head: TimeoutRwLock<BeaconSnapshot<T::EthSpec>>,
     /// The root of the genesis block.
@@ -3216,6 +3216,11 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         let slot = state.slot();
         let proposer_index = state.get_beacon_proposer_index(state.slot(), &self.spec)? as u64;
 
+        let pubkey_opt = state
+            .validators()
+            .get(proposer_index as usize)
+            .map(|v| v.pubkey);
+
         // Closure to fetch a sync aggregate in cases where it is required.
         let get_sync_aggregate = || -> Result<SyncAggregate<_>, BlockProductionError> {
             Ok(self
@@ -3274,7 +3279,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             BeaconState::Merge(_) => {
                 let sync_aggregate = get_sync_aggregate()?;
                 let execution_payload =
-                    get_execution_payload::<T, Payload>(self, &state, proposer_index)?;
+                    get_execution_payload::<T, Payload>(self, &state, proposer_index, pubkey_opt)?;
                 BeaconBlock::Merge(BeaconBlockMerge {
                     slot,
                     proposer_index,