### Search before asking - [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar. ### Description - Introduce new aggregation function types (rbm32/rbm64) and Java factory helpers. - Implement FieldRoaringBitmap32Agg/FieldRoaringBitmap64Agg and register factories via SPI. - Add RoaringBitmapUtils for reusable serialize/deserialize helpers with bounded buffer reuse. - Update aggregation documentation with rbm32/rbm64 usage examples. - Add unit tests for aggregation behavior and AggregationContext mapping. - Add RoaringBitmap dependency and shade relocation in fluss-server. ### Willingness to contribute - [x] I'm willing to submit a PR!
