refactor: add agg_ops.MeanOp for sqlglot compiler

[chelsea-lin]

Fixes internal issue 445774480🦕

[tswast]

e2e failures:

FAILED tests/system/large/blob/test_function.py::test_blob_image_normalize_to_series
FAILED tests/system/large/blob/test_function.py::test_blob_exif - Failed: Tim...
FAILED tests/system/large/blob/test_function.py::test_blob_pdf_extract[False]
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_with_connection[bq_connection_name]
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_df_apply_axis_1
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_df_where_mask
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_resize_to_series
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_dataframe_map_array_output
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_df_apply_axis_1_series_args
FAILED tests/system/large/blob/test_function.py::test_blob_image_normalize_to_folder
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_series_apply_array_output
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_df_where_mask_series
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_dataframe_apply_axis_1
FAILED tests/system/large/blob/test_function.py::test_blob_image_blur_to_folder
FAILED tests/system/large/blob/test_function.py::test_blob_pdf_chunk[False]
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_series_combine
FAILED tests/system/large/blob/test_function.py::test_blob_pdf_extract[True]
FAILED tests/system/large/functions/test_managed_function.py::test_managed_function_series_combine_array_output
= 23 failed, 371 passed, 10 skipped, 6 xfailed, 6 xpassed, 248 warnings in 3654.83s (1:00:54) =

These appear unrelated to this PR.