GFS Data Support in ocf-data-sampler

[siddharth7113]

Description

This PR adds support for GFS (Global Forecast System) data in ocf-data-sampler. Previously, GFS was not recognized as a valid provider, causing a ValueError: Unknown provider: gfs. This change introduces:

• A new GFS data loader (gfs.py) under ocf_data_sampler/load/nwp/providers/.
• Modifications to nwp.py to recognize "gfs" as a valid provider and use open_gfs.
• Ensured compatibility with the existing structure of ocf-data-sampler.

Fixes: #188

Checklist

• My code follows [OCF’s coding style guidelines](https://github.com/openclimatefix/.github/blob/main/coding_style.md).
• I have performed a self-review of my own code.
• I have updated the documentation if necessary.
• I have added tests to ensure functionality works as expected.
• I have checked my code and corrected any misspellings.

[Sukh-P]

Thanks for making a start on this @siddharth7113 think it just needs a few small tweaks and then would be good to go!

[siddharth7113]

Thanks for making a start on this @siddharth7113 think it just needs a few small tweaks and then would be good to go!

Thank you for taking time out to review @Sukh-P , I have made the necessary changes.

[Sukh-P]

Great work adding this in @siddharth7113, thanks!