Support tail workers over rpc

[danlapid]

This allows tail workers over rpc, to test use the following files:
config.capnp:

using Workerd = import "/workerd/workerd.capnp";

const tailWorkerExample :Workerd.Config = (
  services = [
    (name = "main", worker = .helloWorld),
    (name = "log", external = ( address = "127.0.0.1:8081", http = ( capnpConnectHost = "capnp" ))),
  ],
  sockets = [ ( name = "http", address = "*:8080", http = (), service = "main" ) ],
);

const helloWorld :Workerd.Worker = (
  modules = [
    (name = "worker", esModule = embed "worker.js")
  ],
  compatibilityDate = "2024-10-14",
  tails = ["log"],
  bindings = [(name = "log", service = "log")]
);

log.capnp:

using Workerd = import "/workerd/workerd.capnp";

const tailWorkerExample :Workerd.Config = (
  services = [
    (name = "log", worker = .logWorker),
  ],
  sockets = [ ( name = "rpc", address = "*:8081", http = ( capnpConnectHost = "capnp" ), service = "log" ) ],
);

const logWorker :Workerd.Worker = (
  modules = [
    (name = "worker", esModule = embed "tail.js")
  ],
  compatibilityDate = "2024-10-14",
);

tail.js:

export default {
  tail(traces) {
    console.log(traces[0].logs);
  },
};

worker.js:

export default {
  async fetch(req, env) {
    console.log('log from worker a');
    return new Response('response from worker a');
  },
};

bash:

workerd serve deps/workerd/samples/tail-workers/log.capnp
workerd serve deps/workerd/samples/tail-workers/config.capnp
curl http://localhost:8080

[danlapid]

Sadly we don't really have tests to set up two workerd instances and talk rpc between them.

[irvinebroque]

@rohinlohe should be able to see both sides this way, even if there's other work to do on wrangler side