[rust] Add property overlays for rust client Config

### Search before asking

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

### Description

`fluss-rust::Config` does not support applying multiple string properties atomically. Callers have to duplicate property definitions, parsing, field mapping, and validation.

### Willingness to contribute

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

Start by locating fluss-rust::Config and reading the existing property definitions, parsing, field mapping, and validation paths. Trace how configuration values are applied today and identify the relevant tests or entry points. Done means multiple string properties can be applied atomically without duplicating those definitions and validation rules.

Written by the indexing model from the issue text.