Builder profit threshold flag (#3534)

## Issue Addressed

Resolves https://github.com/sigp/lighthouse/issues/3517

## Proposed Changes

Adds a `--builder-profit-threshold <wei value>` flag to the BN. If an external payload's value field is less than this value, the local payload will be used. The value of the local payload will not be checked (it can't really be checked until the engine API is updated to support this).


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

beacon_node/execution_layer/src/lib.rs

@@ -136,6 +136,7 @@ struct Inner<E: EthSpec> {
     proposers: RwLock<HashMap<ProposerKey, Proposer>>,
     executor: TaskExecutor,
     payload_cache: PayloadCache<E>,
+    builder_profit_threshold: Uint256,
     log: Logger,
 }
 
@@ -156,6 +157,8 @@ pub struct Config {
     pub jwt_version: Option<String>,
     /// Default directory for the jwt secret if not provided through cli.
     pub default_datadir: PathBuf,
+    /// The minimum value of an external payload for it to be considered in a proposal.
+    pub builder_profit_threshold: u128,
 }
 
 /// Provides access to one execution engine and provides a neat interface for consumption by the
@@ -176,6 +179,7 @@ impl<T: EthSpec> ExecutionLayer<T> {
             jwt_id,
             jwt_version,
             default_datadir,
+            builder_profit_threshold,
         } = config;
 
         if urls.len() > 1 {
@@ -225,7 +229,14 @@ impl<T: EthSpec> ExecutionLayer<T> {
         };
 
         let builder = builder_url
-            .map(|url| BuilderHttpClient::new(url).map_err(Error::Builder))
+            .map(|url| {
+                let builder_client = BuilderHttpClient::new(url.clone()).map_err(Error::Builder);
+                info!(log,
+                    "Connected to external block builder";
+                    "builder_url" => ?url,
+                    "builder_profit_threshold" => builder_profit_threshold);
+                builder_client
+            })
             .transpose()?;
 
         let inner = Inner {
@@ -238,6 +249,7 @@ impl<T: EthSpec> ExecutionLayer<T> {
             execution_blocks: Mutex::new(LruCache::new(EXECUTION_BLOCKS_LRU_CACHE_SIZE)),
             executor,
             payload_cache: PayloadCache::default(),
+            builder_profit_threshold: Uint256::from(builder_profit_threshold),
             log,
         };
 
@@ -631,7 +643,17 @@ impl<T: EthSpec> ExecutionLayer<T> {
                                 "block_hash" => ?header.block_hash(),
                             );
 
-                            if header.parent_hash() != parent_hash {
+                            let relay_value = relay.data.message.value;
+                            let configured_value = self.inner.builder_profit_threshold;
+                            if relay_value < configured_value {
+                                info!(
+                                        self.log(),
+                                        "The value offered by the connected builder does not meet \
+                                        the configured profit threshold. Using local payload.";
+                                        "configured_value" => ?configured_value, "relay_value" => ?relay_value
+                                    );
+                                Ok(local)
+                            } else if header.parent_hash() != parent_hash {
                                 warn!(
                                     self.log(),
                                     "Invalid parent hash from connected builder, \