Initial work for #3100. Since this is a large change, doing it in parts similar to the AuthManager so it's easier to review and move the existing code around.

Rationale for this change

Introduces the pluggable file format writer API: FileFormatWriter, FileFormatModel, and
FileFormatFactory in pyiceberg/io/fileformat.py. Moves DataFileStatistics from pyarrow.py with a
re-export for backward compatibility. The move is more forward looking and the idea is to keep the stats generic in the future as we add additional formats too.

This is the first part of work for #3100. No behavioral changes; the write path remains hardcoded to Parquet.

Are these changes tested?

Yes. tests/io/test_fileformat.py tests backward-compatible import of DataFileStatistics

Are there any user-facing changes?

No