[helm] Enable external connectivity in the helm charts

### Search before asking

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

### Description

Users should be able to enable external connectivity when deploying using Helm charts.

At the moment, the `CLIENT` protocol binds to the Helm service which is internal to the kubernetes system.

### Willingness to contribute

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

Start by inspecting the Helm charts and how the CLIENT protocol currently binds to its Kubernetes service. Determine the chart configuration and service exposure needed for external connectivity, then verify that a deployment can connect externally while preserving the existing internal deployment behavior.

Written by the indexing model from the issue text.