[Feature] Make bucketAssigner for lake table pluggable

### Search before asking

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

### Motivation

Currently, **LakeTableBucketAssigner** is a fixed **PaimonBucketAssigner**. More data lakes may be supported in the future, so we should add a configuration item to make **LakeTableBucketAssigner** pluggable.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start by locating the LakeTableBucketAssigner entry point and its current PaimonBucketAssigner implementation. Trace how lake-table configuration is loaded and how bucket assignment is invoked; done should include a documented configuration path that selects an assigner while preserving current Paimon behavior, with relevant tests updated or added.

Written by the indexing model from the issue text.