### Description

The new [readiness port](https://www.elastic.co/guide/en/elasticsearch/reference/8.4/advanced-configuration.html#readiness-tcp-port) makes readiness checks on Kubernetes much easier. However the current implementation only allows connections through localhost. TCP probes on Kubernetes are run by the kubelet which is a process running on the Kubernetes node (and not from within the container or the Pod). The [Kubernetes documentation](https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#tcp-probes) mentions it, but it is kind of subtle: 

> For a TCP probe, the kubelet makes the probe connection at the node, not in the pod, which means that you can not use a service name in the host parameter since the kubelet is unable to resolve it.


That means a standard Kubernetes readiness probe cannot be used for Elasticsearch:
```
          readinessProbe:
            tcpSocket:
              port: 8080
```

Instead, only an `exec` probe that runs a shell command inside the container can be used that relies in this case on the presence of the `nc` command in the container image:

```
    readinessProbe:
      exec:
        command: ["nc", "-z", "-v", "-w5", "127.0.0.1", "8080"] 
```



server/src/main/java/org/elasticsearch/readiness/ReadinessService.java
private void setReady(boolean ready) {
        if (ready) {
            startListener();
        } else {
            stopListener();
        }
    }
    /**
     * Add a listener for bound readiness service address.
     * @param listener
     */
    public void addBoundAddressListener(BoundAddressListener listener) {
        boundAddressListeners.add(listener);
    }
    /**
     * A listener to be notified when the readiness service establishes the port it's listening on.
     * The {@link #addressBound(BoundTransportAddress)} method is called after the readiness service socket
     * is up and listening.
     */
    public interface BoundAddressListener {
        /**
         * This method is going to be called only the first time the address is bound. The readiness service
         * always binds to the same port it did initially. Subsequent changes to ready from not-ready states will
         * not send this notification.
         * @param address
         */
        void addressBound(BoundTransportAddress address);
    }
}