[rust] Lookuper / PrefixKeyLookuper: switch to &self lookup for pipelining

### Search before asking

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

### Description

`Lookuper::lookup` and `PrefixKeyLookuper::lookup` take `&mut self`, forcing bindings to hold a mutex across the network `.await`. Concurrent lookups on one instance serialize end-to-end. On the other hand Java pipelines on a single lookuper.

We can wrap encoder in `parking_lot::Mutex`, make `lookup` `&self`, release the lock before `.await`
Also align with bindings(1-5 lines wiring probably required)

### Willingness to contribute

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