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/cli.rs

@@ -957,4 +957,13 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                       This is equivalent to --http and --validator-monitor-auto.")
                 .takes_value(false)
         )
+        .arg(
+            Arg::with_name("always-prefer-builder-payload")
+            .long("always-prefer-builder-payload")
+            .help("If set, the beacon node always uses the payload from the builder instead of the local payload.")
+            // The builder profit threshold flag is used to provide preference
+            // to local payloads, therefore it fundamentally conflicts with
+            // always using the builder.
+            .conflicts_with("builder-profit-threshold")
+        )
 }