Let build2cmake generate metadata.json

[danieldk]

We were generating it from Nix first, but then we need the same logic on the Windows side. So, instead, let build2cmake generate file and let the Nix/Windows builders put it in place.

[mfuntowicz]

LGTM, thanks for making it available for Windows too.

Is there already an example kernel leveraging this somewhere I can give it a quick try on a Windows machine?

[danieldk]

Is there already an example kernel leveraging this somewhere I can give it a quick try on a Windows machine?

I think this would be very useful to test with https://github.com/huggingface/kernels-community/tree/main/rmsnorm with this added to build.toml:

[general.xpu]
python-depends = ["onednn"]