[helm] Support advertised listeners for out-of-cluster clients

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

The Helm chart hard-codes `advertised.listeners` to the pod's in-cluster headless-Service FQDN.

Out-of-cluster clients can reach the coordinator for bootstrap, but Fluss then redirects produce/fetch to each tablet's in-cluster FQDN, which is unresolvable outside the cluster, so all reads/writes fail.

kubectl port-forward doesn't help because the redirect targets are pod DNS names, not the forwarded address.

There is currently no way to run a Fluss client from outside the k8s cluster.

### Solution

Make advertised addresses configurable without changing the default behaviour which is in-cluster only...

### Anything else?

i know there is k8s operator proposal that defers full external-access design to the k8s operator, but this is a default-off chart-level stopgap for the existing behaviour... so i feel it's fine...

let me know if this is something accepted, happy to contribute

### Willingness to contribute

- [x] I'm willing to submit a PR!

Start by locating the Helm chart's advertised.listeners setting and its configurable values, then render the chart with Helm to inspect the generated configuration. Confirm that the default remains unchanged while an external advertised address can be supplied for out-of-cluster clients, and verify the rendered configuration covers the coordinator and tablet endpoints.

Written by the indexing model from the issue text.