Prevent port re-use in HTTP API tests (#4745)

## Issue Addressed

CI is plagued by `AddrAlreadyInUse` failures, which are caused by race conditions in allocating free ports.

This PR removes all usages of the `unused_port` crate for Lighthouse's HTTP API, in favour of passing `:0` as the listen address. As a result, the listen address isn't known ahead of time and must be read from the listening socket after it binds. This requires tying some self-referential knots, which is a little disruptive, but hopefully doesn't clash too much with Deneb 🤞

There are still a few usages of `unused_tcp4_port` left in cases where we start external processes, like the `watch` Postgres DB, Anvil, Geth, Nethermind, etc. Removing these usages is non-trivial because it's hard to read the port back from an external process after starting it with `--port 0`. We might be able to do something on Linux where we read from `/proc/`, but I'll leave that for future work.

Cargo.lock

@@ -601,7 +601,6 @@ dependencies = [
  "tree_hash",
  "tree_hash_derive",
  "types",
- "unused_port",
 ]
 
 [[package]]
@@ -2583,7 +2582,9 @@ dependencies = [
 name = "execution_layer"
 version = "0.1.0"
 dependencies = [
+ "arc-swap",
  "async-trait",
+ "axum",
  "builder_client",
  "bytes",
  "environment",
@@ -2598,6 +2599,7 @@ dependencies = [
  "hash-db",
  "hash256-std-hasher",
  "hex",
+ "hyper",
  "jsonwebtoken",
  "keccak-hash",
  "lazy_static",
@@ -3381,7 +3383,6 @@ dependencies = [
  "tokio-stream",
  "tree_hash",
  "types",
- "unused_port",
  "warp",
  "warp_utils",
 ]