Tighten down API access

### Search before asking

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

### Description

We should audit and make any `pub` struct / fn that are internal only to be `pub (crate)`.

This is important as once released, we cannot change these things without risking breaking backward compatibility as users might use them.

### Willingness to contribute

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