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/src/config.rs

@@ -753,6 +753,11 @@ pub fn get_config<E: EthSpec>(
     client_config.chain.optimistic_finalized_sync =
         !cli_args.is_present("disable-optimistic-finalized-sync");
 
+    // Payload selection configs
+    if cli_args.is_present("always-prefer-builder-payload") {
+        client_config.always_prefer_builder_payload = true;
+    }
+
     Ok(client_config)
 }