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/src/genesis_json.rs

@@ -0,0 +1,42 @@
+use serde_json::{json, Value};
+
+/// Sourced from:
+///
+/// https://notes.ethereum.org/rmVErCfCRPKGqGkUe89-Kg
+pub fn geth_genesis_json() -> Value {
+    json!({
+        "config": {
+            "chainId":1,
+            "homesteadBlock":0,
+            "eip150Block":0,
+            "eip155Block":0,
+            "eip158Block":0,
+            "byzantiumBlock":0,
+            "constantinopleBlock":0,
+            "petersburgBlock":0,
+            "istanbulBlock":0,
+            "muirGlacierBlock":0,
+            "berlinBlock":0,
+            "londonBlock":0,
+            "clique": {
+                "period": 5,
+                "epoch": 30000
+            },
+            "terminalTotalDifficulty":0
+        },
+        "nonce":"0x42",
+        "timestamp":"0x0",
+        "extraData":"0x0000000000000000000000000000000000000000000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
+        "gasLimit":"0x1C9C380",
+        "difficulty":"0x400000000",
+        "mixHash":"0x0000000000000000000000000000000000000000000000000000000000000000",
+        "coinbase":"0x0000000000000000000000000000000000000000",
+        "alloc":{
+            "0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b":{"balance":"0x6d6172697573766477000000"}
+        },
+        "number":"0x0",
+        "gasUsed":"0x0",
+        "parentHash":"0x0000000000000000000000000000000000000000000000000000000000000000",
+        "baseFeePerGas":"0x7"
+    })
+}