Allow multiple filenames per partition when separating by metadata

[ayushdg]

Description

Closes #89

Adds an option to handle scenarios where there are multiple filenames in a single partition when writing with filename.
This is typically true when files are read in with files_per_partition > 1.

Usage

dataset = DocumentDataset.read_json(path, files_per_partition=5, include_filename=True)
write_to_disk(dataset.df, output_file_dir=path, write_to_filename=True)

Checklist

• I am familiar with the Contributing Guide.
• New or Existing tests cover these changes.
• The documentation is up to date with these changes.

[VibhuJawa]

Thanks for working on this @ayushdg , should be helpful across the various modules . Requested some changes around supporting parquet

[VibhuJawa]

LGTM to merge, thanks for working on this @ayushdg