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>

testing/execution_engine_integration/tests/tests.rs

@@ -0,0 +1,16 @@
+#[cfg(not(target_family = "windows"))]
+mod not_windows {
+    use execution_engine_integration::{Geth, TestRig};
+    #[test]
+    fn geth() {
+        TestRig::new(Geth).perform_tests_blocking()
+    }
+}
+
+#[cfg(target_family = "windows")]
+mod windows {
+    #[test]
+    fn all_tests_skipped_on_windows() {
+        //
+    }
+}