Engine API v1.0.0.alpha.6 + interop tests (#3024)

## Issue Addressed

NA

## Proposed Changes

This PR extends #3018 to address my review comments there and add automated integration tests with Geth (and other implementations, in the future).

I've also de-duplicated the "unused port" logic by creating an  `common/unused_port` crate.

## Additional Info

I'm not sure if we want to merge this PR, or update #3018 and merge that. I don't mind, I'm primarily opening this PR to make sure CI works.


Co-authored-by: Mark Mackey <mark@sigmaprime.io>

beacon_node/beacon_chain/src/block_verification.rs

@@ -54,6 +54,7 @@ use crate::{
     metrics, BeaconChain, BeaconChainError, BeaconChainTypes,
 };
 use eth2::types::EventKind;
+use execution_layer::PayloadStatusV1Status;
 use fork_choice::{ForkChoice, ForkChoiceStore, PayloadVerificationStatus};
 use parking_lot::RwLockReadGuard;
 use proto_array::Block as ProtoBlock;
@@ -269,7 +270,10 @@ pub enum ExecutionPayloadError {
     /// ## Peer scoring
     ///
     /// The block is invalid and the peer is faulty
-    RejectedByExecutionEngine,
+    RejectedByExecutionEngine {
+        status: PayloadStatusV1Status,
+        latest_valid_hash: Option<Vec<Hash256>>,
+    },
     /// The execution payload timestamp does not match the slot
     ///
     /// ## Peer scoring