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/build.rs

@@ -0,0 +1,62 @@
+use std::env;
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+const GETH_BRANCH: &str = "merge-kiln";
+const GETH_REPO_URL: &str = "https://github.com/MariusVanDerWijden/go-ethereum";
+
+fn main() {
+    let manifest_dir: PathBuf = env::var("CARGO_MANIFEST_DIR").unwrap().into();
+    let execution_clients_dir = manifest_dir.join("execution_clients");
+
+    if !execution_clients_dir.exists() {
+        fs::create_dir(&execution_clients_dir).unwrap();
+    }
+
+    build_geth(&execution_clients_dir);
+}
+
+fn build_geth(execution_clients_dir: &Path) {
+    let repo_dir = execution_clients_dir.join("go-ethereum");
+
+    if !repo_dir.exists() {
+        // Clone the repo
+        assert!(Command::new("git")
+            .arg("clone")
+            .arg(GETH_REPO_URL)
+            .current_dir(&execution_clients_dir)
+            .output()
+            .expect("failed to clone geth repo")
+            .status
+            .success());
+    }
+
+    // Checkout the correct branch
+    assert!(Command::new("git")
+        .arg("checkout")
+        .arg(GETH_BRANCH)
+        .current_dir(&repo_dir)
+        .output()
+        .expect("failed to checkout geth branch")
+        .status
+        .success());
+
+    // Update the branch
+    assert!(Command::new("git")
+        .arg("pull")
+        .current_dir(&repo_dir)
+        .output()
+        .expect("failed to update geth branch")
+        .status
+        .success());
+
+    // Build geth
+    assert!(Command::new("make")
+        .arg("geth")
+        .current_dir(&repo_dir)
+        .output()
+        .expect("failed to make geth")
+        .status
+        .success());
+}