json: add optional return_file_name parameter

[Sachin-0001]

This PR adds an optional return_file_name parameter to the JSON dataset loader.

When enabled, a new file_name column is added containing the source file name
for each row. Default behavior is unchanged.

Changes:

• Add return_file_name to JsonConfig
• Append file name during JSON table generation
• Add tests covering default and enabled behavior, and ensures other functions are not affected

Motivation:
This helps resume training from checkpoints by identifying already-consumed data shards.

Fixes #5806