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>

Cargo.toml

@@ -39,6 +39,7 @@ members = [
     "common/task_executor",
     "common/target_check",
     "common/test_random_derive",
+    "common/unused_port",
     "common/validator_dir",
     "common/warp_utils",
     "common/fallback",
@@ -74,6 +75,7 @@ members = [
 
     "testing/ef_tests",
     "testing/eth1_test_rig",
+    "testing/execution_engine_integration",
     "testing/node_test_rig",
     "testing/simulator",
     "testing/test-test_logger",