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.

beacon_node/execution_layer/Cargo.toml

@@ -42,6 +42,8 @@ ethers-core = "1.0.2"
 builder_client = { path = "../builder_client" }
 fork_choice = { path = "../../consensus/fork_choice" }
 mev-rs = { git = "https://github.com/ralexstokes/mev-rs", rev = "216657016d5c0889b505857c89ae42c7aa2764af" }
+axum = "0.6"
+hyper = "0.14"
 ethereum-consensus = { git = "https://github.com/ralexstokes/ethereum-consensus", rev = "e380108" }
 ssz_rs = "0.9.0"
 tokio-stream = { version = "0.1.9", features = [ "sync" ] }
@@ -51,3 +53,4 @@ hash256-std-hasher = "0.15.2"
 triehash = "0.8.4"
 hash-db = "0.15.2"
 pretty_reqwest_error = { path = "../../common/pretty_reqwest_error" }
+arc-swap = "1.6.0"