Support reading tiering source data as Arrow RecordBatch #2962

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Description

This issue tracks the tiering-source part of splitting parent task #437.

Today, tiering source reads Fluss log data and converts it into downstream storage formats through row-oriented or storage-specific paths. To support a cleaner and more efficient Arrow-based pipeline, tiering source should be able to read data directly as Arrow RecordBatch.

This work would provide a reusable Arrow-native read path for tiering, and would also serve as the foundation for directly writing tiered data into Parquet in a later step.

Possible scope:

• add a tiering-source path that reads log data as Arrow RecordBatch;
• define the batch lifecycle/ownership clearly to avoid Arrow memory leaks;
• make the Arrow batch path reusable by downstream tiering writers.

This is intended to be one sub-task of #437, while the Arrow-to-Parquet conversion itself is tracked separately.

Willingness to contribute

• I'm willing to submit a PR!