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/errors.rs

@@ -8,6 +8,7 @@ use crate::naive_aggregation_pool::Error as NaiveAggregationError;
 use crate::observed_aggregates::Error as ObservedAttestationsError;
 use crate::observed_attesters::Error as ObservedAttestersError;
 use crate::observed_block_producers::Error as ObservedBlockProducersError;
+use execution_layer::PayloadStatusV1Status;
 use futures::channel::mpsc::TrySendError;
 use operation_pool::OpPoolError;
 use safe_arith::ArithError;
@@ -137,6 +138,10 @@ pub enum BeaconChainError {
     AltairForkDisabled,
     ExecutionLayerMissing,
     ExecutionForkChoiceUpdateFailed(execution_layer::Error),
+    ExecutionForkChoiceUpdateInvalid {
+        status: PayloadStatusV1Status,
+        latest_valid_hash: Option<Vec<Hash256>>,
+    },
     BlockRewardSlotError,
     BlockRewardAttestationError,
     BlockRewardSyncError,