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/client/src/config.rs

@@ -79,6 +79,7 @@ pub struct Config {
     pub monitoring_api: Option<monitoring_api::Config>,
     pub slasher: Option<slasher::Config>,
     pub logger_config: LoggerConfig,
+    pub always_prefer_builder_payload: bool,
 }
 
 impl Default for Config {
@@ -105,6 +106,7 @@ impl Default for Config {
             validator_monitor_pubkeys: vec![],
             validator_monitor_individual_tracking_threshold: DEFAULT_INDIVIDUAL_TRACKING_THRESHOLD,
             logger_config: LoggerConfig::default(),
+            always_prefer_builder_payload: false,
         }
     }
 }