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/beacon_chain/Cargo.toml

@@ -65,7 +65,6 @@ sensitive_url = { path = "../../common/sensitive_url" }
 superstruct = "0.5.0"
 hex = "0.4.2"
 exit-future = "0.2.0"
-unused_port = {path = "../../common/unused_port"}
 oneshot_broadcast = { path = "../../common/oneshot_broadcast" }
 
 [[test]]