[rust] Filter pushdown

### Search before asking

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

### Motivation

Implement filter pushdown for fluss-rust core/client

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

The issue only identifies fluss-rust core/client and does not specify files, tests, or filter semantics. Start by locating the relevant core/client filtering and query paths, then ask maintainers which operations should support pushdown and how to validate them. Done means the agreed filter pushdown behavior is implemented and covered by the project's tests.

Written by the indexing model from the issue text.