feat: implement Index.to_list()

[tswast]

This commit implements the Index.to_list() method, which is an alias for tolist(). This new method provides a way to convert a BigQuery DataFrames Index to a Python list, similar to the existing Series.to_list() method.

The implementation follows the pattern of other methods in the library by first converting the Index to a pandas Index using to_pandas() and then calling the corresponding .to_list() method.

A unit test has been added to verify the functionality of the new method.

Follow-up to https://github.com/googleapis/python-bigquery-dataframes/pull/2105/files#r2369617190
🦕

[tswast]

Notebook failure FAILED notebooks/remote_functions/remote_function_vertex_claude_model.ipynb::remote_function_vertex_claude_model.ipynb - . appears unrelated.

[tswast]

e2e failures:

FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_dataframe_apply_axis_1_array_output
FAILED tests/system/large/blob/test_function.py::test_blob_image_blur_to_series
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_df_apply_axis_1
FAILED tests/system/large/blob/test_function.py::test_blob_exif - Failed: Tim...
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_df_apply_axis_1_complex[column-multiindex]
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_series_where_mask_map
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_series_combine_array_output
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_array_output
FAILED tests/system/large/blob/test_function.py::test_blob_image_normalize_to_series
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_series_apply_args
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_series_apply
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_df_where_mask_series
FAILED tests/system/large/blob/test_function.py::test_blob_pdf_chunk[True] - ...
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_df_apply_axis_1_complex[all-kinds-of-column-names]
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_with_connection[bq_connection]
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_series_apply_array_output

All Python managed functions related. Not related to this PR.