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>

Makefile

@@ -2,6 +2,7 @@
 
 EF_TESTS = "testing/ef_tests"
 STATE_TRANSITION_VECTORS = "testing/state_transition_vectors"
+EXECUTION_ENGINE_INTEGRATION = "testing/execution_engine_integration"
 GIT_TAG := $(shell git describe --tags --candidates 1)
 BIN_DIR = "bin"
 
@@ -123,12 +124,16 @@ run-state-transition-tests:
 # Downloads and runs the EF test vectors.
 test-ef: make-ef-tests run-ef-tests
 
+# Runs tests checking interop between Lighthouse and execution clients.
+test-exec-engine:
+	make -C $(EXECUTION_ENGINE_INTEGRATION) test
+
 # Runs the full workspace tests in release, without downloading any additional
 # test vectors.
 test: test-release
 
 # Runs the entire test suite, downloading test vectors if required.
-test-full: cargo-fmt test-release test-debug test-ef
+test-full: cargo-fmt test-release test-debug test-ef test-exec-engine
 
 # Lints the code for bad style and potentially unsafe arithmetic using Clippy.
 # Clippy lints are opt-in per-crate for now. By default, everything is allowed except for performance and correctness lints.