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/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "execution_engine_integration"
+version = "0.1.0"
+edition = "2021"
+
+build = "build.rs"
+
+[dependencies]
+tempfile = "3.1.0"
+serde_json = "1.0.58"
+task_executor = { path = "../../common/task_executor" }
+tokio = { version = "1.14.0", features = ["rt-multi-thread", "macros"] }
+futures = "0.3.7"
+exit-future = "0.2.0"
+environment = { path = "../../lighthouse/environment" }
+execution_layer = { path = "../../beacon_node/execution_layer" }
+sensitive_url = { path = "../../common/sensitive_url" }
+types = { path = "../../consensus/types" }
+unused_port = { path = "../../common/unused_port" }