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>

common/unused_port/src/lib.rs

@@ -0,0 +1,55 @@
+use std::net::{TcpListener, UdpSocket};
+
+#[derive(Copy, Clone)]
+pub enum Transport {
+    Tcp,
+    Udp,
+}
+
+/// A convenience function for `unused_port(Transport::Tcp)`.
+pub fn unused_tcp_port() -> Result<u16, String> {
+    unused_port(Transport::Tcp)
+}
+
+/// A convenience function for `unused_port(Transport::Tcp)`.
+pub fn unused_udp_port() -> Result<u16, String> {
+    unused_port(Transport::Udp)
+}
+
+/// A bit of hack to find an unused port.
+///
+/// Does not guarantee that the given port is unused after the function exits, just that it was
+/// unused before the function started (i.e., it does not reserve a port).
+///
+/// ## Notes
+///
+/// It is possible that users are unable to bind to the ports returned by this function as the OS
+/// has a buffer period where it doesn't allow binding to the same port even after the socket is
+/// closed. We might have to use SO_REUSEADDR socket option from `std::net2` crate in that case.
+pub fn unused_port(transport: Transport) -> Result<u16, String> {
+    let local_addr = match transport {
+        Transport::Tcp => {
+            let listener = TcpListener::bind("127.0.0.1:0").map_err(|e| {
+                format!("Failed to create TCP listener to find unused port: {:?}", e)
+            })?;
+            listener.local_addr().map_err(|e| {
+                format!(
+                    "Failed to read TCP listener local_addr to find unused port: {:?}",
+                    e
+                )
+            })?
+        }
+        Transport::Udp => {
+            let socket = UdpSocket::bind("127.0.0.1:0")
+                .map_err(|e| format!("Failed to create UDP socket to find unused port: {:?}", e))?;
+            socket.local_addr().map_err(|e| {
+                format!(
+                    "Failed to read UDP socket local_addr to find unused port: {:?}",
+                    e
+                )
+            })?
+        }
+    };
+
+    Ok(local_addr.port())
+}