Add columns support to JSON loader for selective key filtering

[ArjunJagdale]

Fixes #7594
This PR adds support for filtering specific columns when loading datasets from .json or .jsonl files — similar to how the columns=... argument works for Parquet.

As suggested, support for the columns=... argument (previously available for Parquet) has now been extended to JSON and JSONL loading via load_dataset(...). You can now load only specific keys/columns and skip the rest — which should help in cases where some fields are unclean, inconsistent, or just unnecessary.

Example:

from datasets import load_dataset

dataset = load_dataset("json", data_files="your_data.jsonl", columns=["id", "title"])
print(dataset["train"].column_names)
# Output: ['id', 'title']

Summary of changes:

• Added columns: Optional[List[str]] to JsonConfig
• Updated _generate_tables() to filter selected columns
• Forwarded columns argument from load_dataset() to the config
• Added test case to validate behavior

Let me know if you'd like the same to be added for CSV or others as a follow-up — happy to help.