feat: add polars file processor

[kitagry]

Summary

This PR adds support for polars DataFrames alongside pandas DataFrames in gokart's file processors. This PR is very minimum support for Polars compared to #457 and #471

Features

• Multi-backend support: File processors now support both pandas and polars DataFrames
  • CsvFileProcessor
  • JsonFileProcessor
  • ParquetFileProcessor
  • FeatherFileProcessor
• Backward compatible: Defaults to pandas when no type parameter is specified

Usage Example

import polars as pl
from gokart import TaskOnKart

class MyPolarsTask(TaskOnKart):
    def output(self):
        return self.make_target('path/to/target.feather', processor=FeatherFileProcessor(dataframe_type='polars'))

    def run(self):
        df = pl.DataFrame({'a': [1, 2, 3]})
        self.dump(df)  # uses polars-compatible processor

class MyPandasTask(TaskOnKart):
    def output(self):
        return self.make_target('path/to/target.feather')

    def run(self):
        df = pd.DataFrame({'a': [1, 2, 3]})
        self.dump(df)  # Uses pandas processor (default behavior)

Why not #457 ?

In #457, we switch between Polars and Pandas using GOKART_DATAFRAME_FRAMEWORK. However, this implies that we cannot use both at the same time. Since projects often migrate from Pandas gradually, we should allow users to use both Polars and Pandas simultaneously.

This PR support minimum polars features, and then we will implement other useful features!

[hirosassa]

Looks good! 👍

[kitagry]

@hiro-o918 I fixed. Could you review again?