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/http_api/Cargo.toml

@@ -40,7 +40,6 @@ logging = { path = "../../common/logging" }
 ethereum_serde_utils = "0.5.0"
 operation_pool = { path = "../operation_pool" }
 sensitive_url = { path = "../../common/sensitive_url" }
-unused_port = { path = "../../common/unused_port" }
 store = { path = "../store" }
 bytes = "1.1.0"
 beacon_processor = { path = "../beacon_processor" }