Add a flag to always use payloads from builders (#4052)

## Issue Addressed

#4040 

## Proposed Changes

- Add the `always_prefer_builder_payload`  field to `Config` in `beacon_node/client/src/config.rs`.
- Add that same field to `Inner` in `beacon_node/execution_layer/src/lib.rs`
- Modify the logic for picking the payload in `beacon_node/execution_layer/src/lib.rs`
- Add the `always-prefer-builder-payload` flag to the beacon node CLI
- Test the new flags in `lighthouse/tests/beacon_node.rs`

Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/execution_layer/src/lib.rs

@@ -219,6 +219,7 @@ struct Inner<E: EthSpec> {
     payload_cache: PayloadCache<E>,
     builder_profit_threshold: Uint256,
     log: Logger,
+    always_prefer_builder_payload: bool,
 }
 
 #[derive(Debug, Default, Clone, Serialize, Deserialize)]
@@ -241,6 +242,7 @@ pub struct Config {
     /// The minimum value of an external payload for it to be considered in a proposal.
     pub builder_profit_threshold: u128,
     pub execution_timeout_multiplier: Option<u32>,
+    pub always_prefer_builder_payload: bool,
 }
 
 /// Provides access to one execution engine and provides a neat interface for consumption by the
@@ -263,6 +265,7 @@ impl<T: EthSpec> ExecutionLayer<T> {
             default_datadir,
             builder_profit_threshold,
             execution_timeout_multiplier,
+            always_prefer_builder_payload,
         } = config;
 
         if urls.len() > 1 {
@@ -335,6 +338,7 @@ impl<T: EthSpec> ExecutionLayer<T> {
             payload_cache: PayloadCache::default(),
             builder_profit_threshold: Uint256::from(builder_profit_threshold),
             log,
+            always_prefer_builder_payload,
         };
 
         Ok(Self {
@@ -796,7 +800,9 @@ impl<T: EthSpec> ExecutionLayer<T> {
 
                             let relay_value = relay.data.message.value;
                             let local_value = *local.block_value();
-                            if local_value >= relay_value {
+                            if !self.inner.always_prefer_builder_payload
+                                && local_value >= relay_value
+                            {
                                 info!(
                                     self.log(),
                                     "Local block is more profitable than relay block";