Add Description getter method for DataField

### Search before asking

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

### Motivation

In the Java client we have a method `getDescription()` to retrieve the description from DataField

### Solution

Add a getter method in DataField to return the Description

### Anything else?

_No response_

### Willingness to contribute

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