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>

lighthouse/tests/beacon_node.rs

@@ -340,6 +340,21 @@ fn trusted_peers_flag() {
         });
 }
 
+#[test]
+fn always_prefer_builder_payload_flag() {
+    CommandLineTest::new()
+        .flag("always-prefer-builder-payload", None)
+        .run_with_zero_port()
+        .with_config(|config| assert!(config.always_prefer_builder_payload));
+}
+
+#[test]
+fn no_flag_sets_always_prefer_builder_payload_to_false() {
+    CommandLineTest::new()
+        .run_with_zero_port()
+        .with_config(|config| assert!(!config.always_prefer_builder_payload));
+}
+
 // Tests for Eth1 flags.
 #[test]
 fn dummy_eth1_flag() {