feat: Add support for multiple writers for one query

[Vasist10]

This PR adds support for using multiple writers for a single query.
Now users can pass:

• writers — a list of writer names
• writers_parameters — matching list of parameter dicts

The executor now loops through all writer clients and runs each one.
Single-writer configs still work the same for backward compatibility.

issue - #193

I also added tests to confirm:

• multiple writers are created
• both writers run during execution
• old single-writer behavior still works
• config loading supports the new fields

[AndreyMarkinPPC]

Hi @Vasist10, thanks for the contribution. Unfortunately, unit/test_config.py::test_save_returns_correct_data fails due to the proposed change.

Apart from that I have a couple of comments:

• Please do not mark writer property as deprecated.
• writers property can be unified under writer with str | list[str] type. I understand that logically writers are better but from the real usage of garf multiple writers are edge cases so I'd not prefer to expose multiple options for writer/writers at this point.
• keep writer_parameters only - there's no need for writers_parameters. Each writer can be instantiated with same named parameters as well as **kwargs, if anything extra is passed to writer initialization it will be safely ignored.

[Vasist10]

@AndreyMarkinPPC i have unified the configuration for writer...and removed writers_parameters as suggested